Specifically, using Newtonian gravity, I get the following outward push on all particles, using units where G=1, the edges of the cube=2 and the masses are +/-1.
This push repels the positive masses and attracts the negative masses.
I get that the transition to phase 2 begins once r-/r+ gets below 0.458840. The outward pressure on the positive masses and the momentum of the inward motion of the negative masses will continue to decrease r-/r+.
The steady state of phase 4 occurs when F-/F+=r-/r+, which happens when r-/r+ gets near 0.387740. Because this steady state represents a uniform expansion, this will also be the limiting value of v-/v+. It is expected that the accelerations will be small relative to the velocities at this point, and the forces will drop off inversely with the distance squared. This will produce
This model does not feature the dynamics of phase 3 that causes a transient increase in outward acceleration. The observed increasing acceleration of the expansion then shows that we are still in phase 3.