@ Agnibh
The acceleration of a particle as seen from frame S1 has magnitude 4m/s2
This means that the O1 (origin of the frame of reference S1) is moving with an acceleration of 4m/s2 with respect to the frame of reference attached to the particle
Similar is the case of the O2 (origin of the Frame of Reference S2)
Hence you can easily deduce that the acceleration of S2 with respect to S1 is anywhere between 0 and 8m/s2 depending on the direction of the respective acceleration.
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