4 weeks Portugal, Spain, southern France

I would generally start in the south and work my way north. It's probably not going to make a huge difference, but every little bit of relief could help, might as well take advantage of that.

Transportation: The areas you're looking at in Spain and France are well-connected by train. Oddly for us tourists, that isn't the case in Portugal. While there are frequent high-speed trains between Lisbon and Porto, there are no really good land options between Portugal and Spain. Most people fly.

High season: yes and no. While it's high season in southern France, July is definitely low season in Andalucia and that is because of the extreme heat; triple digit temps are frequent. Madrid is about the same. Instead, consider flying to Valencia, then train to Barcelona, train to France. Ryanair, Tap, and Vueling all fly to Valencia from Lisbon, Ryanair also flies there from Porto.

Doing all three countries will take some planning. I would allot one week to Portugal, two weeks to Spain and one week to Provence.

Suggest starting in Portugal. If you can fly into Porto do so, if not do Lisbon and then move up to Porto. There is a nice high speed train between the cities, but don't take it both ways. There are some great places between, Sintra, Obidos, Bathila Cathedral, Navarre and more.

Book the In Patio Guesthouse (book early, it fills up quickly) in Porto. It is the BEST B&B we have ever experienced.

Getting from Lisbon to either Madrid or Seville will not be so easy. There was an overnight train to Madrid, not still there is now.
Renting a car is a problem, since the drop charge crossing into another country is prohibitive. Suggest flying into Seville. Check flying from Porto, if not available or expensive then from Lisbon.

Seville is the jewel of Southern Spain, suggest 4 nights there. Move from there to a night in Cordoba then a couple of nights in Granada. Then head to Madrid. Don't miss the Prado Art Museum. Suggest day trips to Toledo and Segovia. Then take the high speed train to Barcelona. Don't miss Monserrat.

After Barcelona move up to France. Cannes, Monaco and Nice are nice as well as the village of St. Paul de Venice.
Arles, Avignon, Aix-en-Provence and Pont du Gard (intact Roman aqueduct). If time see the Rhone Valley up to Lyon.

Summer will be hot and more crowded and May would be better than July.

Peak season in France starts for most on 4 July, when prices rocket and everywhere gets fully booked, so if you can get there the week before, I would start there for 7 nights. The tap really does turn on and off for tourist numbers in France for the peak 6 weeks and the rest of the year.

Spain and Portugal don’t have the peak troughs in the same way that France has, so do these later. Seville and Madrid are going to be unbearably hot throughout the month.

From your list of places, it sounds as though 1 week or so for France, 2 weeks for Spain and a week for Portugal would be the right balance. Naturally, you could also spend the full month in a single country.

Southern Spain will most likely be a hellish inferno by late July, even Madrid will be unbearably hot during our numerous summer heat waves, that will definitely limit your sightseeing time to early morning and evening. Save yourself the trouble...

Instead, from Oporto, I'd recommend heading into Northern Spain, rather than turning south. Cross into Galicia, the Celtic region of Spain, filled with lush forests, old monasteries, and Roman ruins. Then cut across the mountainous regions of Asturias and Cantabria, before ending up in the culinary hub that is the Spanish Basque country. From there cross into the French Basque Country, then cut through the historic Languedoc region, before ending in Provence.

Summer is the perfect time to visit Northern Spain (aka Green Spain), it's totally unique compared to the rest of the country and with significantly less tourists hordes then you will find in Sevilla or Barcelona, that time of year.

Before you finalize plans to spend a substantial part of your time in Seville (especially) and Madrid, take a look at the actual, day-by-day weather statistics for July 2019, 2018, etc. on the website timeanddate.com. Seville is typically shockingly hot at that time of year and the area around Madrid is likely also to be way up in the "Miserable" zone. Southern France can truly bake at that time of year. You have options. The stretch of northern Spain from Galicia to the Basque Country and Pyrenees is usually much, much cooler.

Please don't think that you have hot weather at home and Europe will be no worse. Unless you and your family are employed as gardeners, golf course groundskeepers, etc., you don't spend all that much time at home in the non-air-conditioned outdoors. If you live in a hot climate, you have an air conditioned home and an air conditioned car that you use to get to air conditioned work places and stores. Not everything in Europe is air conditioned, including a lot of museums that do not hold paintings. Casual cafes and restaurants will not necessarily be air conditioned. That's why on the hottest days you see so many people choosing to sit outside under big umbrellas; at least there may be a breeze. You can do that, too, if you don't mind probably being surrounded by a lot of smokers.

Yes, it gets hot in Spain in the Summer. I suppose it depends on what you are used to on the weather. When I lived in Saudi Arabia, working for the US Army in the 80s, Spain was cooler than Saudi.

Also, we live in South Georgia and the heat in the Summer is comparable to Spain. Some people can only travel when their children are out of school and must do long tours of Europe in the Summer.