For what it’s worth I think the 4th substitute which is being trialled in certain tournaments is a good addition to the rulebook.
Just to qualify the 4th sub usage, it is only allowed during extra-time in tournament knock-out games. If three subs can be used in 90 minutes, why not one more in 30 minutes, there is a certain symmetry here.
Personally I hope it doesn’t open the floodgates to having unlimited substitutions, rather like ice-hockey, but for once FIFA seems to be trying something proactively.
One point to be raised is that of two-leg knock-out games. If extra-time is needed at the end of the second leg, then the home team for the second leg will obviously be playing a significantly greater part of the two matches in front of their home supporters. I’m not sure if a 4th sub is appropriate here; be careful FIFA.