In the line with 4.Nf3, White specifically is returning the pawn for major positional trumps. If you want to hold on to the pawn, you have to play 4.Bf4, but then tactics loom and this is what Black is hoping for.
4.Nf3 is actually stronger, but it does return the pawn to Black.
Whats best dealing with this opening as white. I've been struggling trying to keep the pawn with Nf3.