4.b4?

Just looking into some Morphy games, couldnt quite figure out this pawn sac move, could some enlighten me? Thanks!

Indeed, the idea is to distract one of the light pieces from attacking the d4-file, and if one of them takes the pawn, then white wants to play 5. c3 and 6. d4 and get a powerful center, while his opponent only has two pieces and one pawn developed. The problem is, after 5. c3 Ba5 the c3-pawn ends up pinned, and 6. d4 exd4 is not as favorable then - white often ends up sacrificing the c3 pawn as well to accelerate the attack.

The gambit is surprisingly strong, and is one of the reasons many players avoid playing 3...Bc5 and employ two knights defense instead (3...Nf6).

Evans was a Welsh sea captain who invented this gambit in around the 1830's. It took a long time for players to work out how to deal with all the white threats and eventually around 1900, or just before, world champion Lasker demonstrated that the best way is to give back the pawn for a free game and a fairly equal/slightly better for black position. Previous world champion Steinitz played several ugly games with black against the Evans.