Don't give a shit about the amp.

I need a 4-string bass that is particularly good for slapping (I intend to learn pretty much nothing but Primus songs).

I intend to pay as high as 300 bucks for the bass itself.

I don't give a shit about brands or looks, just help out a completely clueless amateur guitarist

Up to $300!!

Most decent guitars/basses start around that range, and by decent, I mean...a step above beginner guitars. Yamaha BB's are pretty decent.

http://bass-guitars.musiciansfriend.com/product/Yamaha-BB415-5String-Bass?sku=519167

And slapping is more of an issue of setup/technique than the bass itself, just FYI.

bought a yamaha rbx 170 for 250$

it's everything I could possibly want so far, which in reality isn't much