specifics in Spear fighting combat

[Kineas]

I think you are incorrectly linking the "customary" part of the phrase to the depth of 4. Clearly, to me, he is referring to the individual units "customary", and probably argued over for honor's sake, position from right to left along the battle line. To Xenophon this would have been very important. I don't believe Xenophon is telling us that there is anything coustomary about the 4 rank depth. It is possible that this was a depth used against non-hoplites, there is good reason for this, but it is just as likely that the depth was a facade to make the army look more impressive to his audience.

Perseus is down, so I can't check my work and histemi is not my best verb, but the important part of the sentence (man, you are making me sight-read--my Greek ain't that great) says (apologies for bad latin transliteration)

Ekeluese de tous Ellenas (He ordered the Greeks) hos nomos autois (in the manner customary to them) eis Maxen outw taxthenai kai stenai, (to form lines and to stand just as into battle).

I wouldn't want to bet my life on this translation, and I just got a valauble lesson on how much I rely on computers for verbs like histemi (stenai is a 2nd Aorist active infinitive, or so I'm guessing and god help me).

My point is that if "customary" referred to the location of the sub units, I'm pretty sure the sentence would have a referential indication, one of the many Greek markers like "I'm about to say more" like the men de construction (since he goes on to discuss the formation side by side.) As a final linguistic nail, he says "taxthenai" in this sentence and "taxthesan" in the next, which I recall as a nice way of linking the two--same verb, see, so the two thoughts are linked. so...to form lines... formed lines in 4 ranks" are linked thoughts.
I'd be delighted to have another Greek reader check my work. Greek is not my first or 2nd language!