Hi Roger,
Four countries is no joke; though the countries are similar, but each is different and unique in its own way. I guess since it's your first time you wish to cover more countries; though personally my wife and I would probably just do France and a bit of Spain since we just did Italy and Switzerland for our honeymoon. In fact, we don't mind doing France in one month; it is such a big country and there is so much to see and do.
Last May, we had the option of doing France, Germany, Switzerland, but decided in the end to stick to one country; so we were in France for 19 days. Even then it was quite rushed and we merely scraped the tip of the iceberg.
Just make sure you don't drive more than the four hours that you are used to. So you need to plan your route carefully. Mountain roads take about twice the time to travel through. Both times, I rented manual cars; but automatic cars are available too. You need a while to get used to driving on the right hand side of the road, so just drive slowly at first.
The Alps are fine; it's like driving up to Cameron highlands. No problems in Summer. If vehicle breaks down, contact the rental car company.
You can easily see stopovers; e.g. petrol stations, small towns, supermarkets, etc. No problem, and it's part of the fun; having lunch in a quiet small town where few Singaporeans have ever ventured.
Speed limits are about 130 km/h; if you drive at 100 km/h you will find that you are almost the slowest vehicle there, apart from the bicycles. Haha. Get used to it. But the drivers are generally more polite than in SG; they don't tailgate.
Trains may or may not be cheaper; you don't need to navigate, but you need to know how to transfer trains, buy the tickets, look after your luggage, and find accommodation near the train station. Car rental gives you a lot more flexibility and you can explore those places unreachable by trains; but you need to navigate and plan your route. Personally I always drive; it's part of the enjoyable experience for me. Can stop as and when you like to take pictures; but you can't ask the train driver to stop.
July is Summer, so unlikely to be cold unless you are in the mountains where it could get chilly. Take note it is the peak period so you should book your plane tickets and accommodation early.
In fact, if you are doing only France I could plan something for you; but then that takes the fun out of going F&E for you. Part of the fun is in planning, reading guide books, and choosing exactly where you want to go, and even sometimes eschewing the popular and crowded tourist spots for undiscovered and hidden gems.
