You're trying to run AE on just 4 gigs of memory? That's your problem right there -- you need 8 gigs to get it to run right, and it's a lot better to have 16.
It really does help to check out the system requirements before you just assume a poece of software is going to work, y'know.....
Thanks Dave. I agree with my MBA 4g memory is too small. However, sorry I did not write down this, my friend's iMac shows same freeze problem when reviewing. That iMac has mid 2010 3.06 GHz Intel Core i3, Memory 16GB 1333MHz DDR3, Graphics ATI Radeon HD 4670 256MB. Memory 16GB could be enough. Probably Graphic memory 256MB is too small?
Your graphics memory shouldn't matter.
How long is your composition?
What is in your composition?
Are you still facing this problem? If not, let us know how you solved it. If so, please let us know so we can assist you further.