Yes, it is "enough" to run AE, but whether it will be enjoyable working, is another matter. You definitely should at least double the amount of RAM if you can afford it or go even higher within what your motherboard supports and your purse allows for...
4GB is enough to run After Effects, but it's far from the optimum amount. It's an especially low amount if you'll be running any other programs at the same time.
It depends on what kind of work you're doing. Let's say that you'll be working with HD video and maybe 2K film. In that case, you'd ideally have 3GB installed per processor core. So, if you have a four-coure computer (which has eight processor cores with hyperthreading), you'd have 24GB of RAM. That's exactly how my home computer is set up. If you're working with 4K film, you'd have more RAM; for SD TV, you could have less.
See this page for some information about memory settings and performance: