I'm not sure about the total thrust of SLS Block II (Crew Stack). So the math could be wrong. Since the Atlas 5 has a version where 4 SRBs are used, is this also possible for SLS?
Such a thing might be possible but it would involve a major redesign of the rocket. All of the forces on the central core would be different so the core would need to be reinforced. As SpaceX found when they produced Falcon Heavy from Falcon 9 it is not a straight forward issue.
There are many other issues as well, not show stoppers but they all add complications. When twin side boosters are exhausted and released they symmetrically fall away to either side. But with four one will have to be ejected with an upwards component or the release will need to happen in two stages with a rotation in between.
It would also mess with the aerodynamics of the whole vehicle, the point it reaches Qmax and the throttling arrangements. All of this can be dealt with but the complexities may not be worth it. Rockets aren't Lego.