It depends on:
1) What you need - if you have a bad GPA, be cautious b/c you can't afford B's.
2) Your background (stated above)
3) What else you're doing - volunteering, working, etc.
4) Which classes they are - pre-reqs are much more time-intensive than most non-science courses.
5) When you're taking the MCAT - try to take it easy & concentrate your efforts on the MCAT that semester.
I just dropped from 4-->3, I volunteer 12 hours a week, and am studying for the MCAT. I couldn't see it being possible to take 4 classes and do all that - even with 3 + MCAT you have to cut corners on occasion.
Most advisors will tell you to start slow - it just sucks b/c most classes are in sequence.