When DS was 4 (he's 6 now) he was still very into his Matchbox cars and die-cast airplanes....would line them up on the rug, have them "climb" up the stairs, create huge parking lots of them. With the airplanes came some "runway" pieces and he would make runways and have them take off...look for "Hot Wings" airplanes, they are about $4 each. For some reason, he would re-enact scenese from the Lion King or Nemo with his stuffed animals and cars....for instance, the blue corvette would fall into the gorge where the hyenas were stampeding, or the fighter jet would be riding the East Austrailian Current, or whatever.
I can't remember exactly when the dinosaur obession began, but, well, we're still obsessed. The Dollar Store had great plastic dinos for super cheap and we got some accompanying books and there is a cartoon, Land Before Time, that isn't too scary. At age 4 he was differentiating between plant eaters and meat eaters. We have also had an outer space and shark phase.
He was big into puzzles at age 4...the floor puzzles with big pieces. I got some on Ebay for pretty cheap. But mainly the cars, airplanes and dinosaurs, all of which cost maybe a dollar, maybe 3, each.