I agree with Cath. Make sure they are close to being ready before you enter them. A lot too depends on what level they were before you took them on - presumably they were not beginners ?? ! Have they taken exams before? if not, they have much less idea of what is involved.
One other small point - two months is quite a short time (in my book) to get to know a new student and his or her working style. If I were in your position, I would still be building up a picture of their overall weaknesses and strengths.
If they are progressing brilliantly, and if they can play the pieces, scales etc, and you have established that their ear-tests and sight-reading are well up to the grade, then go ahead. But if any of these raises a question mark in your mind, I would recommend exercising a little caution. You can always give them lots of other pieces to play if you decide that autumn would be better to take grade 4. It's not a race after all!
Good luck with your teaching