Well I'm new to writing fugues myself, but maybe my comments can be helpful anyway...
First of all for terminology's sake I think you have more than an exposition there. An exposition basically ends when the last voice enters.
Your entrances seem to be coming in good pitch order (Tonic-dominant-Tonic-Dominant), so that's good.
From what I know of fugues though, I think they should come in with each entrance in in outer voice, so that the entrance is clear. Your order is alto-soprano-bass-tenor, but IMO the tenor voice gets a little lost. I would change it to alto-soprano-tenor bass.
As far as the harmony, I'm having a hard time figuring out what your progression is. The dificulty with counterpoint is to keep each voice melodically independent while bringing them together for a clear, functional harmony. Make sure that you have a clear harmonic progression planned out. My greatest successes so far have been when I let one voice develop itself melodically and use that voice to suggest the harmony.
My biggest struggle so far in my attempts has been to write a good, complimentary counter-subject. I think your might be improved on as well. Mainly because I don't think it presents a really clear harmony (see point above) and also because it seems to get a little lost and muddled. I think the counter-subject has to have a really high melodic content. As I said before I suck at that part, so take this comment as empathy, not arrogance. You may want to try to make it very motivically solid.
I think you have a good start. Keep working on it, with or without my suggestions. I'm sure you will do fine.
By the way, if you haven't read J. Lee Graham's crash course yet, you should. It's very useful.
http://www.youngcomposers.com/forum/A-Cras...gues-t4302.html
Wether or not my attempts sound good, I had a great time trying this exercise, and am glad to see others trying it. Good luck!