The problem with "Support" as a 4th role has largely been the way it was implemented. Rift and other games chose to make Support effects be group/raid wide and often of a fixed percentage. This leads to problems with scaling that cause designers to pull their hair out in large tufts. Principle among them is how do you make it so the buff/debuff effects of the Support are large enough that you can't just replace them with a different pure role.
I think these efforts have been somewhat misguided as they really don't match up well with the focus of the other roles which can be summarized as Bar Managers.
Tanks keep the enemy focused on them so that their health bar is the only one effected, Healers keep the player health bars topped off, and DPS are focused on dropping the health bar of the target. Tanks have evolved somewhat to use extensive types of self-shielding and self-healing to maintain their own health bar, in some cases "shields" are used to add secondary health bars and to a certain degree everyone manages their Mana / Rage / Energy bars as a fuel resource. If we want Support to be widely viable then it needs to have some similar sort of bar management as its bread-n-butter.
In classic EverQuest the Enchanter and Bards did provide mana regen as a sort of bar management and were used as means to keep fuel supplies topped off. Unfortunately this was mostly done from the point of view of keeping static regen abilities active rather than being used as a "mana healer". EQ2 have classes that provide mana increase as a directed effect rather than a persistent regeneration along with directed cooldowns...but this was often regarded as more of a necessary evil than a true team asset.
I could see a Support role working in terms of a directed or short duration regen "boost" that has a similar mechanic to a heal or a shield but which provides a substantial +gooder effect to whomever is effected by it. Imagine a Mage spec built around Amplify and Dampen magic where instead of being fixed buffs they acted as a shield providing extra damage when used on a DPS class, extra healing throughput + Spirit when used on a healer, and extra damage mitigation when used on a tank but only as an expended resource that gets used up as the affected character uses their abilities. The Support character would be expected to pump up the tank or healer during periods of spikey damage or funnel power into the DPS during burn phases coordinating with specific classes to know who to pump up when in order to make best use of the added oomph.
In addition to augmenting other players the Support could assume a distinct debuffing role where again the debuff is expended from target actions rather than just being a managed duration. With the prevalence of movement based dodging of ground effects the Support role could additionally provide bonuses like "cast while moving" as a directed effect ALA the Medic in TF2 or even cooldowns that set up a stationary bubble on the battlefield that makes those standing within protected from ground effects of various types
Ultimately I think something needs to be done to break the logjam of DPS overpopulation and while adding a 4th role might not solve every problem I think it would go a long way to clearing up some of the backlog...especially for those classes which are shoehorned into ONLY the DPS role.