Personally, I would not accept such a W4 as it is clearly not valid (back dated). If there was no valid W4 in place before the 2019 "checks" to be back dated, then it must be (for 2019) Single, 0 allowances, and no other adjustments.
Not saying there is a good chance of getting caught, just that the slope is slippery. This forum is public, so it makes no sense to post anything "slippery" here, regardless of the chances of getting caught.