Transverse diffusion (flip-flop) is the movement of:

In the case of a membrane phospholipids, two types of movements are possible: lateral shift i. e. movement from one place in the bilayer membrane to the other within the same leaflet; and transverse diffusion is the movement of the phospholipids from one side of the membrane to the other, which is a relatively long process.

Hence, the correct option is 4.