Bon dia!
I have no experience with the 4ed of Gurps. But I agree totally with Copeab and Spielmeister about the 3rd Edition. I use the 3rd Edition rules to play games in ancient/medieval historical settings. From my point of view, no house-rules needed in this settings. For World War II scenarios I'm studying the possibility to use GURPS 3rd Ed, too. In fact, Spielmeiser has convinced me in a previous thread
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I think that the 3rd edition set or rules is very solid (and extensive). And the advantages/disadvantages/quirks system is a very good way for the players to imagine his/her own character and make it different from the others. You will find that every player has a clear vision of his character previously to the first gaming session.
Ah! And don't miss any chance to get the GURPS Character Builder! A great software to help the GM making characters and knowing the game!