In FN-2187's 4 man squad there is also FN-2199, FN-2000, and FN-2003. Shouldn't the numbers be consecutive like FN-2000, FN-2001, FN-2003, and FN- 2004?
Ok, so the Stormtroopers are given their numbers very early in training, and then matched up with other troopers to create squads. But the numbers are not consecutive because they do not necessarily work well with those troops. Even then, some troops in the squad may not be able to pass tests, live fire exercises, vigorous training, or may be moved up or down like to sanitation duty or officer training(Source for training and officer training: Star Wars Rebels, and Before the Awakening). Also, because of Captain Phasma's direct oversight and FN-2187's top scores as a soldier his 4 man squad was probably a little more elite.