How would they fix PLD 4.0?

[Sarcatica]

Except that tanks actually swap as much as possible to get the most benefit, in the best speedruns. If you see a12s, there are 4 swaps from start till finish. This is currently the best optimization that pretty much kills your idea on PLD, mainly because it's never needed and won't have much use.

Aside from that, the easiest is obviously to rework PLD's kit to allow more and bring its DPS to match with the rest.

Let's do a simulation on a12s with your idea. The general idea is to let WAR tank the most, WAR has to stack a lot of CDs back to back for Heat tank buster. 1st phase to add phase generally no problem and requires no further help from PLD. The problem starts from after adds till the end, WAR has to save Vengeance and all the other small CDs for 1st stasis and Holmgang for 2nd stasis. During timegate WAR will only have ToB and RI, very unlike to survive the tank add so he has to tank boss. Vengeance and other small CDs will be up on 1st Inception and Holmgang on 2nd Inception before killing Alex Prime. It seems all nice and good EXCEPT that PLD loses a good chunk of damage from Shield Swipe every now and then. PLD can also tank a good portion of the tank busters with frequent swaps for more proc damage, ie. Stacking Rampart and Sentinel on 1st stasis tank buster and swap WAR for HG on 2nd stasis. Then there is DRK being better than PLD by a miles in a12s mainly because without Reprisal, the big difference is their DPS gap, Reprisal makes it better. Skin Wall DM slightly better than Rampart Sentinel. 2 small differences big enough are DRK will have Skin DM VS PLD with Rampart, during timegate. DRK will have Wall DM on 1st Inception VS PLD with Sentinel if they tank boss during Timegate. PLD overall needs more help to survive which doesnt allow healers to DPS.

TLDR: your idea doesn't do much on a real situation.

TLDR2: if you aren't into absolute optimization, PLD is fine. It can clear just fine and you can optimize with it. DRK is considered better on the upper realm of the top speedrun mostly due to the maximum damage output potential. Same goes to WHM and casters no doubt.