Laura,
I really wouldn't worry about the 4yr vs. 5yr problem. Like every other college freshman, I too pondered many other majors before deciding on a science major with courses that paralleled many med school requirements. I thought that my premed advisors were terrible, rude, and very unhelpful and was therefore forced to find many things out the hard way. How far from graduating are you? You need to take the MCAT/apply for med school an entire year before you graduate. Concentrate on doing your very best (A's, and some B's, but C's ((one or two)) won't kill you) on the medical school required couses. If you don't know what those are, get a list in your premed advising office. There are certain courses that are highlighted in the MCAT so make sure you take those BEFORE your Sr. year and BEFORE the MCAT if at all possible. Those courses include physics, chemistry, organic chemistry, and biology/physiology. There's also a reading section on the MCAT that many people find hard to prepare for. However, I think the best way to prepare for that is to begin NOW reading classical novels by classical authors. My favorite: Alexander Dumas' The Count of Monte Cristo. This is an excellent book, and others like it, because it has many characters and subplots that make you think and recall.....that's what the MCAT reading section is all about.
I'll stop with that advice for now. It's a starting point. I'm full of info so let me know if I can help with any other questions, specific or general...
kimberly