So, this thread will be a little weird, because I just solved my problem while writting it lol. However, in order to help all the people which seems to suffer from the same problem, I let a small message.

I suffer the infinite launcher loop (launcher fallout 4 -> Launcher start -> click play -> "Prepare Fallout 4" -> Launcher start -> click play and so on...).

Tried many of the solutions find around the internet : update / downgrade graphic driver, run as admin or in windows 7 compatibility mod for steam / launcher or game exe, disable windows UAC, reinstall game, rename game .exe and launcher, delete . ini preference file, etc.

Nothing seem working, so I was just writing a post here. To make all my tests complete before posting. I also try disabling both my Comodo firewal and my AVG antivirus.

And it works !!!!!!

I appear that AVG prevent F4 to start properly after some further test (probably a recent upgrade).

Hope it will help all the other people suffering from that.

lol, that was the quickest post ever...thanks for the info, and enjoy your game.