I think its very difficult to predict in terms of proportions, as it totally depends on the course and where you're applying, some places would be full now and others take applications until July. Try not to worry about it, you've submitted an application that I presume you're happy with, and all you can do now is wait. You'll probably have to wait up to 3 weeks for a response, but a lot are much sooner. Then you'll have an informal interview which could be in a group then individual, or just individual. Then they'll either tell you there or you'll get a letter shortly after.
People do get rejected, but as long as you are what the university is looking for you should be fine. Try not to be too nervous in the interview, they aren't trying to catch you out. And if you do get rejected (it happened to me), there is always a plan B, and it might be better for you. Just try not to over-think about it, don't put it on too high a pedestal, just be confident in your own abilities. The fact that you've been working for 4 years could be an advantage as well. Good luck!