Create an alphanumeric sequence

I want to create a 4 character alphanumeric sequence. The sequence is to be like this: a1a1-z10z10. If it's a number, increment 1 until you reach 10, then move to the next column, if it's a letter, increment a until z, then move to the next column. Excel's autofill feature doesn't seem to work well for me due to the letters involved, so if someone has a macro handy that would be great.

Thanks in advance for any help.