Evans finished with a hat-trick and substitute Mark Bradley also netted as Wales hit back with three in the final 11 minutes after falling 2-1 behind.
"But that's what we've been working on this week and the display didn't surprise me, although I'm certainly delighted with it.
"What you have to realise is this is a very good bunch of young players. They did extremely well because France worked very hard to stop us playing.
"Four appears to be our lucky number because we scored that many in Belfast, in Sweden and against Bosnia-Herzegovina last Saturday. Bosnia-Herzegovina were no mugs and neither were the French."
Two of Evans' goals came from penalties while Yoann Gourcuff also struck from the spot for France.
Dimitri Payet claimed the French side's other goal.
Making the victory all the more remarkable was the fact that the Wales senior squad, who face Germany on Wednesday, have six under-21 qualified players with them in Frankfurt.