Move seperate bytes into a word

[tazntex]

How can I combine these bytes into a word size variable? I am using only four bits of the bytes "0-3" and would like to place them into a word in such a way that "Z" holds A= 0-3, B= 4-7, C=8-11,and D=12-15.
Example:

A=%00000001
B=%00000010
C=%00000011
D=%00000100

Z=0100001100100001

I was thinking of using Aliases in the manual but not sure if that was a way to do it.

I really didn't want to do this:
z.0=a.0
z.1=a.1
etc.
Thanks