Too much reengineering. You would have to redesign the whole rest of the airplane.
The B-52, while it really cool aircraft, has a lot of design flaws that are mitigated by compromises and flying technique. One of the flaws is it doesn’t have very good rudder authority at low speed, like during takeoff.
On takeoff at low speed and heavy weight in the 4-engine configuration, loss of a single engine on one side would cut the thrust of that side in half, and the flight controls would not be able to compensate for the resulting yaw, resulting in loss of aircraft.
With four engines on each side, loss of a single engine is only 25% of the thrust on that side, and there is still enough rudder authority to have a chance at counteracting the resulting yaw.
Four engines (if you could make them fit under the wing, which is unlikely) would require a bigger rudder, but that would require a bigger tail, and that would require redesigning the whole airplane to make it strong enough to keep the tail attached. If you dig back into the B-52’s history, you find out that it has a record of mishaps due to losing the tail because of turbulence in flight.