I had kinda the same question, Bob. My string was: CCYYMMDDOP520MMDDCCYY
I wanted the first 8 characters stripped away so that I ended up with : OP520MMDDCCYY
Support suggested the following
New name mask could be:
{STRING(Remove|{NEWNAME()}|0|8)}
so, for you, it looks like it should be:
{STRING(Remove|{NEWNAME()}|0|4)}
I will try this tomorrow.