4-Bros. Random Block
From the Super Mario Wiki
A 4-Bros. Random Block is a special variety of Random Block in Mario & Luigi: Partners in Time. It is a yellow ? Block with a circle of light orbiting it. It requires a piggyback jump to hit. When struck, the block will spawn two red and two green blocks, with the letters A, B, X, and Y on them. Each member of the Mario quartet will stand under his appropriate block. The circle of light will go in a random block, and that Mario brother must jump to produce ten coins. The circle will then move to another brother's block, and the process continues, eventually getting faster and faster until a brother fails to hit the block.
It's possible to strike the block up to fifty times before it automatically stops, but it will stop earlier if a brother takes too long to hit his block or the wrong brother strikes his own block.