Dressing for April in Spain?

I’ve worn sandals and knee length skirts in Andalucia in March, so the same could be assumed for April. But you will likely find cooler temps in Madrid. You really just need to look at the forecast and be prepared to edit your packing accordingly.

It will be chilly in April. Personally, I wouldn't take any shorts or sandals.
Spain Weather in April
Depending on where you're staying during your trip to Spain, you're likely to experience vastly different weather no matter what time of year you visit. However, the climate around the entire country is usually comfortably warm throughout April, getting gradually warmer as the month progresses.

Average High Average Low
Madrid 66 F (19 C) 44 F (7 C)
Barcelona 65 F (18 C) 50 F (10 C)
Seville 75 F (24 C) 53 F (12 C)
San Sebastian 62 F (17 C) 49 F (9 C)
Canary Islands 72 F (22 C) 61 F (16 C)
The southern Spanish region of Andalusia experiences overall warmer weather than the northern portion of the country, which tends to be cooler and also rainier. If you're visiting San Sebastian in the Basque Country or Galicia in the northwest, make sure you have waterproof clothing or an umbrella.

Spain temps vary a lot in different areas. Layers will be needed.
Seville/Andalusia will likely be hot in April ( a native I took a tour with said the April festival is always sweaty hot).

Spain tends to be much more casual about dress in churches than, say, Italy.
Enjoy Spain!

Don't worry about what the locals wear, you're not going to Spain to pretend to be a local so wear what you feel comfortable in. I'm in shorts and t-shirt in Andalucia in April despite most Spaniards dressing like it's the depths of winter, if I dressed like they did I'd be so hot and uncomfortable. In the evenings it's cooler so I'll wear a very light jacket.

April does tend to see rain and the forecast for Easter shows a lot of rain and some wind for parts of Spain so check the forecast before you go and pack accordingly.

I've been to Granada, Sevilla, Madrid a few times in April; never been to Barcelona.

Days in Madrid can still be chilly and nights can be cold, especially if there's a breeze. Remember that you'll be out and about very late if you're on Spanish time, so expect to be outside in the colder times of the day.

Sevilla was fairly warm each time I went in April -- short sleeves during the day, long sleeves needed in the evenings.

Granada, in my experience, was also short sleeves during the day (lots of sunshine) but fairly cold at night (Granada is at a higher elevation).

Obviously your experience may vary, so check the weather before you leave and adjust accordingly. Also, I am from Vermont, so spring in Spain feels very warm to me; you might have different tolerances.

For footwear, I usually bring one pair of walking shoes and my Aetrex flip flops, which I've had for years and are super comfortable for me. I've logged many flip flop miles in April in Sevilla and Granada, so you might consider sandals if the forecast looks good. Even if it's not sandal weather, I always bring one extra pair of shoes so that I have something to change into if my feet get wet or if they just need a switch.

For clothes, I usually pack a couple of short sleeve shirts, a couple of long sleeve, a lightweight zip hoodie (tech fabric, not a big, heavy cotton athletic hoodie), a puffer jacket, and a super lightweight rain jacket that can layer over everything if needed. A couple or three pairs of pants and maybe a skirt, and I can handle any weather that might come along.

Enjoy your trip -- Spain in April is beautiful!

These days I do not worry so much about what is typical. Last year proved to me that all seasonal expectations are off and so the ten day forecast is the only thing I rely on.
As far as how much clothing to take, what is your plan in terms of laundry? I typically rent an apartment with a washer and build in time for it to dry, as dryers are a rare occurrence. You could also make use of laundry service.
For a ten day trip, I only take a carry-on size rolling suitcase.
You will need some warmer clothing for Montserrat, so I would just let that be the layers you will use for cool mornings and evenings anywhere else. I would take a really nice raincoat, and some light wool layers.

Just look up each city you’re going to a couple weeks from your trip. That will give you a good idea of what to expect where. Weather prognosticators are no better for Spain than they are in your home town. Everything is subject to change.

By April in southern Spain I would be in my summer gear with a jumper or light jacket for the evening. It’s hot for those of used to a Northern climate so an awful lot depends on where you live. My daughter has been in a sundress paddling in the sea before while the local children are in coats and woolly tights!

Here now, In Alicante. Spending three months in Spain, started Feb. 1 -15 in Madrid, then Feb. 15 - March 16 in Sevillas (with trips during that time to Granada, Cordoba, Cadiz), then in Valencia from March 16 - 24, and now in Alicante. Leave and go to Barcelona on Thursday, and staying for the month of April. Please learn from my packing mistakes:

1. It’s colder and cloudier and drier than usual all over Spain this winter/spring. It’s definitely spring but can be very cool in the morning, in the evening and if it gets cloudy and windy. Often, when sun is shining, it is warm in the afternoon for a few hours.
2. You need layers. Most days I have on a short sleeve or tank, followed by a long sleeve, then a sweater/sweatshirt. And even then in some cases I”m wearing my thin down coat on top of all of that when it’s particularly cool (cold).
   
3. I brought two pair of sandals and have not worn them yet. My husband wears his Keen sandal, but always with socks.
   
4. We both brought shorts and have never worn them yet. Long pants are needed.
5. I brought 5 dresses, all sleeveless, with the intent of wearing layers over that, and wearing my sandals. I had a couple of days like that in Sevilla, that’s it. I have worn my skirts after buying leggings that match here in Spain. So if you like the leggings with sneaker look, a dress or skirt might work for you.
6. I brought 2 pair of pants because I thought I’d be wearing my dresses, skirts and shorts. Wrong. I have bought three more pairs of pants here in Spain. Why? Because it’s pants and jacket weather.

Come, enjoy, and do not expect it to be hot until you are in Seville. And even then, I’m not sure how hot it’s going to get.

For the past 7 weeks, no daytime temp has exceeded 74 degrees Fahrenheit.

Firstly... "Spain" is not a city or a county.... due to location and orography climate varies wildly from one end to the other. Thus, Madrid is likely to be fresher than Seville for example. Saying temperature is running X now is a bit useless... where exactly? As an example, in Barcelona now we wake up at 10ºC and run up to 18ºC at noon, Montserrat -which is just an hour away by car- wakes up at 3ºC and jumps only to 10ºC at noon. In April, most likely, add 3-4ºC to today's temperatures -at least here in Catalonia.

Regarding "dress codes" -again I am talking only about the leg of your trip here in Catalonia- I wouldn't worry too much. It's unlikely you want to go to a black-tie dinner, right? Other than that, "smart casual" is the norm for the evening and "casual" for the rest of the day. As per religious buildings, the only place where you could find certain restrictions is the Cathedral of Barcelona where short shorts or sleeveless t-shirts might not be allowed -depending on the guard you find at the entrance. Other churches (ie Sagrada Família) don't have any particular dress code, other than the obvious (for example, you can't pretend to enter in your bikini!).

While on a sunny day, you might want to walk the promenade by the beach, or even sit on the sand and sunbath, it's not warm enough for a dip into the sea -unless you come from a cold region yourself and a sea which is 12-14ºC is what you're used to.

I am so glad I asked you all! I have much better insight as to what to expect for weather and how to pack. Thanks so much everyone for sharing the snapshot of Spanish weather and your experience!