Product Sort Order - Restrict to A-D, E-H etc

Hi

What I've been asked to do is to be able to sort alphabetically but restrict to certain groups - e.g A-Z sort but restrict to groups of A-D, E-H, I-L and so on.

I know how to sort alphabetically but how can I add the other level?

We'd also like to do a similar thing by Value - e.g.