Now days $4000 won't buy a very big engine, so apparantly you are looking in the 25hp range. I just looked up the NADA value of the '86 Johnson 150GT I bought new, 26 years ago. It cost about $7000 then, it's book value is now $210 (no, I did not leave out a zero). It will still push my bass boat to 57mph though.
I disagree with going carburated. A newer fuel injected engine starts easier (especially if it is cold outside) and is about 4 times as efficient burning fuel. I get about 1. 5 mpg. If you get an older engine find an older mechanic, the young ones can't fathom a non-computerized engine.