Can a creditor go after a deceased's 401k account?
The account is already in the beneficiary's name.
No, they cannot do this. These funds pass outside of the probate estate. And only estate assets are subject to the claims of creditors. Upon the death of the contributor to the plan the designated beneficiary takes over the right to these funds and they are not liable for any debts of the estate. This is much the same as if they had been a beneficiary under life insurance policy which are also not subject to any such claims.
Adding to & modifying a prior response, under Missouri law the proceeds from a qualified retirement plan are generally exempt from creditors of the former employee who created the plan, except for a QDRO (a divorce decree) or a plan created within 3 years of a bankruptcy filing that is treated as fraudulent. The Missouri statute is 513.430, and like most statutes is very complex. So, there is no black or white answer to the question.