I'm curious why you're not interested in using uint8 to hold your data. Do you have a specific goal in mind (regarding uint4) other than storing small integers efficiently?
If you're actually after a 4-bit representation of some integer, you can use dec2bin like this approach: http://stackoverflow.com/questions/2687678/integer-array-to-binary-array
I don't think making your own uint4 class will be worth the effort.