The accelerations of a particle as seen from two frames S1 and S2 have equal magnitude 4 m/s2
The acceleration of the particle is 4m/s2 with respect to two frames S1 and S2, this means the acceleration of frame S1 is 4m/s2 with respect to the frame of reference that is attached to the particle and same acceleration will be for the frame S2.
So, the acceleration of frame S2 with respect to frame S1 may be from zero to 8m/s2 and this will depend on the direction of motion of both frames.