specifics in Spear fighting combat

[Kineas]

I think it is at least possible that we should be wary of all mentions of depth, from Aristophanes to Xenophon.

Xenophon uses words carefully (all Greek writers do) and often leaves things out intentionally--he's writing for an informed audience of his friends. When he says that a group formed 4 deep, he may in fact be saying "we were so good, we didn't need the expedient of cowards--a deep formation."
Likewise, when he has the Thebans form in great depth, he notes first that by doing so they endangered all their allies who had agreed on a different depth (a VERY important line for this argument you guys are all in, because it meas that everyone used different depths and had to be standardized before battle--go figure) and second, I get the implication (hey, this's just me) that by being in great depth, they're complete cowards. Which is kind'a his point, over and over. Xenophon hates the Thebans.
Contrast this to Diodorus Siculus account of the battle.

LV. Now on the Lacedaemonian side the descendants of Heracles were stationed as commanders of the wings, namely Cleombrotus the king and Archidamus,1 son of the King Agesilaus, while on the Boeotian side Epameinondas, by employing an unusual disposition of his own, was enabled through his own strategy to achieve his famous victory. [2] He selected from the entire army the bravest men and stationed them on one wing, intending to fight to the finish with them himself. The weakest he placed on the other wing and instructed them to avoid battle and withdraw gradually during the enemy's attack. So then, by arranging his phalanx in oblique formation, he planned to decide the issue of the battle by means of the wing in which were the elite. [3] When the trumpets on both sides sounded the charge and the armies simultaneously with the first onset raised the battle-cry, the Lacedaemonians attacked both wings with their phalanx in crescent formation, while the Boeotians retreated on one wing, but on the other engaged the enemy in double-quick time. [4] As they met in hand-to-hand combat, at first both fought ardently and the battle was evenly poised; shortly, however, as Epameinondas' men began to derive advantage from their valour and the denseness of their lines, many Peloponnesians began to fall. For they were unable to endure the weight of the courageous fighting of the elite corps; of those who had resisted some fell and others were wounded, taking all the blows in front. [5]

Pardon the long quote, but the full description shows how little emphasis (none at all) is on DEPTH. It is the ELITE who decide the battle, and the lesser soldiers are sent to retire gradually and take up time. The better hoplites are "dense" but in no place are they "deep."

The Greek word for density used here is puknotês, the same word that Arrian uses for "close order.'

I am completely aware of the dangers of this contention, but I think we have to take all of Xenophon's views of depth with a grain of salt--and ditto Aristophanes. I believe that men are placed deeper when they are "weak" ie afraid, and thus, deep formations become a sign to the reader (in Xenophon) of cowardice. certainly in the quote I put up from Aristophanes, the intimation is supposed to be that the Athenian men are NOT brave. (We're talking Lysistrata, for heaven's sake!)

: )