I don't do biology, so I don't know anything about the different modules and how long/difficult they are, but assuming units 1 and 4 are worth the same and you only resit one of them, wouldn't you have more chance of improving on unit 1 because the AS modules are easier? From what you've said, it would also be slightly less work than unit 4, so you'd have more time for your other 2 modules. If unit 4 is worth more though, it'd be best to resit that instead. Saying that, if you want to resit both and you think you can handle the workload, go for it. It's your decision. As for whether you can still get a C overall with an E at AS, you can, but you'll need a B at A2, and without meaning to sound harsh, it seems quite unlikely that you'll improve by 3 grades by only resitting 1 or 2 modules, especially as A2 is harder.