The most obvious "4th" role of any kind is simply a "buff/debuff" role to begin with. However that doesn't even fit with the current system.
The existing setup is "Rock Paper Scissors", when you add a 4th, you basically have to de-balance something. So rock-paper-scissors-dynamite adds dynamite, which blows up rock, burns paper, but is only defeated by scissors cutting the fuse.
So for this context, Rock is Tank, Paper is Healer, and DPS is Scissors. Tank's damage is healed by healer, healer generates enmity that attracts enemies, DPS kill enemies, tank attracts adds by generating enmity on purpose (thus takes damage.)
To slot in a 4th role, results in either being too powerful or too weak. So to use the dynamite analogy in RPS, this 4th role ends up being redundant for two roles (eg DPS and Tank) and results in not being healable. Or if it's redundant for DPS and Healer, they end up being glass-cannons. Or redundant for Healer and tank, being unable to do any damage at all.
So Healer+Tank = Sentinel (only has shields, and uses only self-healing) , DPS+Healer Chemist (that makes bombs or medicine buffs), DPS+Tank Power Armor (Think D.Va or Samus, where they can sacrifice the tanky armor with a large hitbox for single-use heavy DPS for a few minutes before the armor can be summoned again, but until, then, they do weaker DPS, but have a smaller hit box.)
As you can see, for most content this would require a rebalance, as the Sentinel would replace the tank and healer, so no need of a healer, so they can have 3 DPS in the party. The Chemist would replace the healer or a DPS for a "green DPS", and the Power armor role would replace the Tank role, or a DPS role.
Rebalancing would simply not be doable since the preferred setup would be Sentinel+3 DPS. At least for content that you've out-geared. The other two "4th roles" would be little more than rebranded DPS.
Unless 5.0 came out with a new "required" mechanic to deal with that can only be dealt with by this 4th role, it's still really just redundant with something that a job already does. Really the thing to do in 5.0 would be to make the existing roles more rigid so that players can not simply outgear a duty, they have to focus on playing their role, not numbers.