Assuming that Equestria adopted a Westerosi-like politico-military structure, with 4 Wardens to each command a quarter of the nation's total military force, who would those Wardens be?

Shining Armor and Cadence are the obvious choices for duel Wardens of the North, given that he's a former member of Celestia's Queensguard and she's an alicorn princess.

I'd tentatively nominate Twilight as Warden of the South, since she's also an alicorn princess and the unofficial leader of the bearers of the Elements of Harmony. She also lives the furthest south of any major authority figures in Equestria, since they've yet to establish if there's any sort of pony population in the Badlands.

Celestia and Luna would be the Wardens of the East and West respectively, mirroring the rising and setting of the sun.

Sounds right. But then again, I didn't quite understand what exactly these titles encompassed anyway.

4137688 ohhh I like this

4137712

"Warden" is a title given to the Lords Paramount of the four geographic extremities of the Seven Kingdoms (The North, The Reach, The Vale of Arryn, and The Westerlands). The Lords Paramount were independent kings in the days before the Seven Kingdoms were united by the Targaryens.

4137789
Okay, so it's more a historical title, grounded more in tradition than in actual political significance.

4137802

Yes, but the Wardens do have real power. The Lord of any Great House that resides within a particular region is expected to be a bannerman to the Lord Paramount of said region, with the Lords Paramount in turn being beholden to the King of the Andals and the First Men, who rules over all of Westeros south of The Wall. The seat of the Warden's Great House considered the capital of that region (Winterfell, Highgarden, the Eyrie, and Casterly Rock).

It works. The south also seems to be newly colonised (Appleoosa, dodge junction, etc) so it would make sense that the youngest princess rule over it.