Yes, what you say is correct. We don't really care about shifting by
more than 15 because that would just zero out the value (or fill it
with 1's if it was a sign extend).
Does the fact that I can only shift by amount the given by 4 bits mean that
this ALU can only shift from 0 to 15 bits to the right or left? Basically, I
should not be able to do a 16 bit shift on this ALU?