Same here for the most part, but I never got what I wanted out of the assigned textbooks. Who in their right mind would tell their students to buy Boron and Boulpaep's Textbook of Medical Physiology? I read a textbook, but it was Costanzo's Physiology (the text, not the BRS review). Alberts is a *great* text, so I thought about buying it just to have on my shelf. If the OP is really familiar with cell biology from UG, he/she doesn't really need the text for med school.
Oh and to go back to the original question, the 4th edition is adequate for medical school cell bio. There were a couple of topics (minor points) that I couldn't find in the 4th edition, but I never looked to see if they were in the 5th, I looked them up on the internet (and ultimately they were too small to be tested). From the Amazon description, it looks like they revised a few of the subjects that you'd cover (like apoptosis, cell cycle). If you're a PhD student or plan to do a lot of future basic science research, you might benefit from the 5th edition updates. If I had to choose for myself, I would stick with the 4th. Cheaper and more aesthetically appealing as it gathers dust upon my bookshelf along with all those other hardcovers I foolishly purchased during my first year.