Retroactive reinstatement of Safe Harbor nonelective - 3% or 4%?

I'm not entirely clear on this. Say in 2020, a SH Nonelective plan was amended to remove the SH. Now in 2021, the employer wants to reinstate the SH for 2020. This is ok. But, is the SH now required to be 4% (since it is retroactive to the prior year) or can it be 3%, since it is reinstating the 3% nonelective that already existed but for the prior amendment?

edited to remove an inconsistency in original post

4%

Thanks John - that's what I was getting out of 2020-86, but I was afraid I was missing something...