Lego set for a 4-year old?

I'd recommend buying the "generic" set. The ones where you have to build specific models etc. are usually hard for this age.

(By the way, if you happen to live around the area of Nehalim, there is a Zol-Po branch there that sells a 1000 part lego set for ~100NIS. Cheapest i've seen).

I think buying a more difficult lego set was better for me, like the battleship I made when I was five. I got bored with the big ones, maybe because it got old, but Star wars or that bionicles sets were cool because they opened up a world of imagination.