A few W's are not the end of the world. As long as your science GPA and MCAT scores are solid, nobody is going to really notice those W's.
The reality of admissions is that they have thousands of application to go through...nobody is going through your application with a fine tooth comb. I'm on the adcom at my medical school, and when we get the applications from AMCAS, it's just a huge long list of classes and grades. I have yet to see the number of W's a student had being an issue.
If your dropping classes every semester, that's a different story. 4-5 are not a huge deal in the grand scheme of things. Class load, major, difficulty of classes, number of W's...none of that stuff matters. Science GPA and MCAT are the only thing people really look at in terms of your transcript. If those are good, nobody is going to notice a couple W's.