Are SEP contributions included in 401(a)(4) testing?

A client sponsored a SEP during 2021 and started a 401(k) plan with new comp in 2021 as well. They made some deposits into the SEP already, I know that these contributions have to be combined for limitation purposes but am not clear if these contributions can be used to satisfy 401(a)(4) testing. 

You need to look at the SEP document. Most (but not all) don't allow the SEP and a 401(k) to exist at the same time.

The 401(k) plan cannot count contributions made to the SEP when running the 401(a) tests for the 401(k) plan. Good: owner contributions in SEP aren’t in the test. Yay!  But, too bad: employees allocations are also not in the test, not for helping with top heavy, they don’t count toward the minimum gateway, etc.