I am on vacation for the Fourth, and so am away from my library and on mobile, so this response will unfortunately be a bit truncated.

I would seriously question the assertion that during the fourth century (or at any point in Late Antiquity) barbarians were regarded as superior to domestic recruits. Barbarians were certainly considered attractive recruits, though for reasons that were not new to Late Antiquity. Germanic barbarian society had a strong martial tradition, and was centered around subsistence agriculture, both of which would have conditioned prospective recruits to the demanding physical environment of army life, as well as made the inducements of military service attractive. It is worth noting that similar conditions drove domestic recruitment. During Late Antiquity, domestic recruits predominately came from poorer, peripheral regions of the Empire, such as Illyria.

Barbarians were thus viewed as attractive recruits. However, in the field armies, only around a third of recruits were drawn from non-Romans. Whether through volunteers or through conscription, the army continued to be staffed predominately by domestic recruits. The idea that the army became "barbarized" by excessive barbarian recruitment has been throughly debunked by modern scholars.

The Roman army of the fourth century continued to be an effective institution. This is the consensus of modern scholarship, though the individual efficacy of constituent parts is debated. High levels of training and disciplined continued to be maintained. Quality military equipment was produced in state-run arsenals. The logistical system which undergirded the army continued to be maintained, overseen by the bureaucracy and funded by the wealth of a pan-Mediterranean empire. It was by no means weak or depleted.

For an overview of foederati during the fourth century, see my response [here] (https://www.reddit.com/r/AskHistorians/comments/30h8qg/at_the_decline_of_the_western_roman_empire_did/cpsga1n); relevant section is right after the edit. Essentially, it was not until the mid fifth century in the West (and never in the East) that the Romans began really hiring independent barbarian groups en masse. This was due to the erosion of the tax base (in part due to the loss of North Africa to the Vandals), which prevented the Western Empire from funding the army at traditional levels.

Please don't hesitate to ask any follow ups. As I mentioned at the beginning, I'm on vacation, so I may have to wait to answer some things if I need to check a reference.