The data is stored after each flip-flop on the "Q" output, so there are four storage "slots" available in this arrangement, hence it is a 4-bit Register. To give an idea of the shifting pattern, imagine that the register holds 0000 (so all storage slots are empty). As "Data In" presents 1,0,1,1,0,0,0,0 (in that order, with a pulse at "Data Advance" each time—this is called clocking or strobing) to the register, this is the result. The right hand column corresponds to the right-most flip-flop's output pin, and so on.