I've seen a lot of people, such as Keith Hackett, suggesting that if they couldn't have VAR at every game in the fourth round they should have waited until the fifth round to introduce it for each tie.
But I don't really see what advantage that would serve - the likes of Everton would still have been knocked out of the cup in the fourth round and surely the aim for most supporters is to win the cup, not to get to the fifth round!