Every mainline title in the Dragon Age franchise has featured a new protagonist than the previous game, exploring the universe of the series in various locations and fresh perspectives. It seems likely that Dragon Age 4 will continue this trend when it's eventually released; however, it could be argued that the story would be stronger by instead allowing a previous player character to return. There will be spoilers up ahead for Dragon Age: Inquisition as well as its Trespasser DLC.
The first release in the Dragon Age series, Dragon Age: Origins, starred an unvoiced protagonist in the form of the Warden. Their story, however, was almost entirely wrapped up by the ending of Origins; so when Dragon Age 2 was released in 2011, it instead followed the journey of a new character, Hawke. Although Hawke made a return during the main questline of Dragon Age: Inquisition, they didn't serve as the player character. At the moment, it's unknown for certain whether or not Dragon Age 4 will continue the pattern - though it most likely will.
Little is definitively known about the story of Dragon Age 4 currently, but there are a few hints towards what the game will be about. It seems like Dragon Age 4 will take place in Tevinter, and it's been all but confirmed that Solas - Dragon Age: Inquisitions' companion-turned-antagonist - will be returning as the primary villain of the title. The Trespasser DLC for Inquisition set this plot element up, and the trailers that have been released for Dragon Age 4 thus far seem to be heading in that direction as well.
The betrayal of Solas is one of the biggest twists in Dragon Age: Inquisition, especially for players who had a close bond with him or even romanced him through the course of the game. The Inquisitor's final confrontation with him during the Trespasser DLC builds upon the tension of this reveal, setting up Solas' villainy as a personal matter to most players as well as the Inquisitor. Although Solas' actions impact the whole of Dragon Age's world, the personal angle helps to make it a more engaging story point as well as a more compelling threat.
Putting Dragon Age 4's story - and Solas' defeat - in the hands of a new character would arguably be diminishing the Inquisitor's role in the franchise. In-universe, there's seemingly no real reason for the Inquisitor to back off and not take charge of the efforts to locate and stop Solas, and it could also be argued that their story will largely remain unresolved until the plot thread is wrapped up. Although it's highly probable that the Inquisitor will appear in Dragon Age 4 to at least some extent, like Hawke did in Inquisition, it would probably make more sense for them to return as the protagonist even if it means breaking the pattern that most other Dragon Age titles have followed.
