300hp under $4k is a long strech, I doubt you can pull it off. The best engine to build cheap would be a B18A/B, though.

To be honest, if you are on a budget, you may want to look into early 90's turbo DSM's (4G63 Turbo) or 87-91 RX-7 Turbo II's (13BT) because both can be made to 300+hp much easier then a Civic, on the stock engine even. Just a thought.