Prevailing Wage Offsets and Testing

I have a client with a document that allows prevailing wage contributions to offset employer matching (non-safe harbor) contributions. What test applies to the portion of the prevailing wage contribution used to offset a participant's employer match, ACP or 401(a)(4)?

It can't be ACP so it must be 401a4. I'd have to noodle a bit as to exactly how though.

It is extremely difficult to find definitive guidance in the prevailing wage world. Without any specific guidance I will likely test under 401(a)(4).

Thank you.