Hey CaptainJack02,
I too have the third edition. If you would like a better answer to the question you pose, check the examkrackers website
(specifically: check the bulletin board questions; I'm pretty sure I saw it in there).
From my opinion: the things that changed the most were:
1. formatting. the 4th edition is alot prettier, uses more colors and has each lecture broken down into sub-sections.
2. This is the important one: since the mcat shifted its bio section around a bit, the 4th edition has more information about genes; there's some more info about dna technology, the lac-operon etc.
Since the guys at examkrackers are so nice, they posted the revised gene section as a pdf on their website (I would recommend downloading this and scanning through it; I cut and pasted all of the stuff that I didn't have into a different pdf and printed that)
I hope I've helped a bit, and I could be wrong about some of the changes; I'm actually going to go back to the site and check the bulletin board right now!!
Cheers,
Silenthunder
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