Only take Spanish 4 if one of these two items applies to you:
1.) You really, really like learning another language and all the work that goes along with it.
2.) You plan on taking AP Spanish 5.
The AP class looks good on college applications, and if you score decently on the AP exam (a 3 isn't that hard), you can get some credits toward your college degree before you even enter college. I got 5.3 units toward the 120 I need to graduate by getting a 3 on that AP. And, the $88 or whatever is a lot cheaper than paying for an equivalent college course.
EDIT: I've forgotten most of the Spanish I've learned, but I don't care. I had a really nice teacher who didn't give a lot of work, but it also meant I didn't learn as much. Yet, I managed a 3 on the AP, so you shouldn't have too much trouble.
Also, at my university, you only need 3 years of high school Spanish to pass out of the foreign language requirement. So, check your intended-college's requirements before making a decision.