I think 4 should be across the board regardless. He game is too fast for a ref to see everything correctly. Again, just look at this weekend, an on the ball incident should have been a red but was ignored.As an American that sees all the perils of overuse of video replay/challenges:
VAR should only be used for:
1) Did the ball go over the line for a goal (we already have this with goal line technology)
2) Offside goals. A goal is scored that VAR proves was offside - think John Terry in the 97th minute
3) Penalties given that are not penalties (VAR would prove guy got ball first, etc)
4) Away from the ball red-card offence type fouls that the ref doesn't see
I think they need to take away the ref reviewing it and put it in the hands of the people watching the replay. Refs have hubris, not many will be excited to overturn a terrible call they made