You can make a 4k go pretty hard. Get your head ported and matched manifolds, exhaust system, then get a blower off a 4agze, and use a 1.75"SU to draw through. Simple but effective and not too much of a headache to tune as there is heaps of SU needles and jets to chose from to get it running smooth. Just run about 8 pound and you'll have something fun to drive.