Will it be super rainy or moderate this time of the year?
April is one of the driest months in the Highlands..the odds are on you side
no one knows what the weather will be like until the day before, and even then it can change on the day
prepare for everything and anything
I live in Seattle, so Scotland weather was familiar to me. In April, I took a lightweight windbreaker. I wore layers and it gets windy, so my hair was flying all over. But I did not need a heavy winter coat. If you hike, you might want a heavier coat.
Scotland's weather can be split roughly into East and West. The West gets more rain but is milder than the East which is colder but also drier. This is more true for central Scotland than the Highlands where weather tends to be a bit more volatile. But always travel prepared for all weather - Scotland is famous for having 4 seasons in one day !
Whenever you travel in Scotland you want to think layers. And then you want a very good waterproof jacket. I like one with a hood so I can keep my hands free. If you are planning on do some walking in the countryside you can think about rain pants. I got some for $10 when I was there and they made all the difference for my hiking.
We were in Scotland in Mid-April and we had snow flurries! Just wear layers and a pull down knit hat. However it was very pleasant most of the time. There are plenty of indoor places to visit if it sprinkles.
I always check weather.yahoo.com for average temperatures and rainfall.
Statistically speaking, April-May are Scotland's dryer months. That said, if you are in Scotland for a week or more, expect to get wet. We visited the Highlands for a week in May, statistically one of the dryer months of the year. We were drenched one day, just missed getting drenched another day, got drizzled on a couple more days and had some blue sky days. Bring rain gear.
Keep in mind that Scotland in not statistically speaking homogeneous. As an example, Western Scotland:
Rainfall is generally well-distributed throughout the year but there is a marked seasonal variation. The frequency of Atlantic depressions is normally greatest during the winter but, unlike other areas of the UK, Scotland tends to remain under their influence for much of the summer too. Autumn and early winter are the wettest seasons, especially from October to January, and spring and early summer is normally the driest part of the year, especially from April to June.