"3" means that there were 3 late onset cues (i.e., 3 late onset X's).
"4" means that there were 4 late onset cues (i.e., 4 late onset X's).
"All" is the 0 late-onset condition, and this means that all the X's were cued (i.e., all the X's disappeared for 100msec and then reappeared), and this was a signal to participants that all the items in the search display should be searched for the target.

The data shown here are only for correct responses (i.e., HITs or CORRECT REJECTIONs), and are taken from Burkell and Pylyshyn's first and second reported experiment.