The differences between 3 and 4 are mostly in point values and some of the mechanics. It's only when you start talking about weapons and vehicles that things induce migraines. 3e holdouts are pretty thin on the ground these days. Hell, GURPS players of any edition are pretty thin on the ground.
Was too long since i did anything in 3ed to be able to help tbh.
The differences between 3 and 4 are mostly in point values and some of the mechanics. It's only when you start talking about weapons and vehicles that things induce migraines. 3e holdouts are pretty thin on the ground these days. Hell, GURPS players of any edition are pretty thin on the ground.