If a is downward acceleration of 4 kg block, the upward acceleration of 1 kg block must be 2a.
If T is tension in each part of string, then equations of motion of the two blocks are 4a=4g−2T...(i) 1×2a=T−1g...(ii)
or 4a=2T−2g...(iii)
Adding (i) and (iii), we get 8a=2g,a=4g ∴ Acceleration of 1 kg block =2a=2g upwards.