A block of mass M rests on a table. It is fastened to the lower end of a light, vertical spring. The upper end of the spring is fastened to a block of mass m. The upper block is pushed down by an additional force $3 m g,$ so the spring compression is $4 m g / k .$ In this configuration, the upper block is released from rest. The spring lifts the lower block off the table. In terms of $m,$ what is the greatest possible
value for $M ?$