You want
GURPS Update, the free publication which addresses differences between 3rd edition and 4e. You've spotted one of the big ones (no PD in 4e), but there are a number of more or less subtle ones as well. If you can get a bunch of free 3rd edition books, do so. They tend to be useful to the extent that they're
not rules-heavy, and a bunch of the really important books have 4e replacements where all the conversion and update work has been done for you (for example,
Martial Arts and the entire
-Tech series). However, there's a wealth of rules-light worldbooks and genre books which are 90%+ usable as-is, and the rules material is easily converted. And for books where there's no 4e equivalent, like
Vehicles, the 3rd edition book is better than nothing and is likely to give results which will at least get you in the right ballpark.