If a plan covers only union employees, then the plan is deemed to pass 401(a)(4) testing, and no further IRC 401(a)(4) testing is required.
If a plan covers both union and non-union employees, the portion covering union employees is mandatorily disaggregated from the portion of the plan covering non-union employees. This means that the union portion of the plan is tested separately from the non-union portion of the plan. The portion covering union employees is deemed to pass IRC 401(a)(4) testing.