There are lots of four cylinder options out there, depending on what you want to do. Olds Quad Fours are great in light rods. For a more nostalgic look track down a Pontiac, International, or Ford "slant four" (literally 1/2 V-8 blocks, usually the right bank -- use the same head/pistons/rods and many other parts of the V-8s). Not many speed parts, but the header is easy and good used cams can be re-ground.
There was a thread here about Chevy and Ford marine fours. The blocks are made heavier due to the constant speed stress of marine engines. That's the only application for the Ford "slant four" -- it was never used in a land vehicle as far as I know. The Chevy marine four is based on the old Chevy four first used in the early Chevy II, IIRC.