The 4C is probably not too bad a mouthpiece for you right now, probably. It's pretty easy to play. Move your reed strength up a half-step, then down a half-step and see if either is easier for you to play.
Make sure your horn is leak-free, take it to a tech and have him or her take a look at it and get it sealing well.
As for rhythm, the BEST thing to do is get a metronome, and use it WHENEVER you practice. Do your scales to it, make up rhythms and repeat them over and over with it... over time your beat will improve (pun not intended).
Also, a private teacher would certainly be a lot of help.
You could also try recording yourself playing along with some Jamey Aebersold backing tracks (you can find them at your music store or at Jazzbooks.com) and you can hear what you are doing right and not right and make some adjustments. MOst people (including myself) are much better listeners than they are players, so becoming your own listening critic can help.