You can not pledge your 401K as collateral. It is not legal. The reason is because you can not be forced to give up your 401K if you default on the loan
doesn't your 401k plan have online access? applying for a loan should be there and easy
It will be on your statements or in your plan documentation.
If you don't keep any of that stuff (not a smart move), your HR department will also have that contact information.
Your 401k statements will have that information.