I bought a used 4-person tub for $200 off Craigslist, and while the shell and tubing seem fine, the first thing (and hopefully last) thing I see wrong with it is the motor, which looks shot. The tub has one 4-HP motor (with an SE of about 1).

So, if I have to flat-out replace it, should I stick with a 4 hp, or could I go lower?

The tub itself has no more than 20-22 jets, and it doesn't have the bubble maker feature.

Having that many jets, it would be a good idea to stick with a 4 HP motor. The amount of piping it takes to push a strong jet of water will take a pretty serious motor to accommodate that demand.