Comparing what was in the PDZs adjacent to the Taurian Concordat at the end of the Third Succession War with the Operation RAT deployment tables, there were a few changes in deployment:
Islamabad PDZ: Islamabad CrMM stayed on Pierce, Fighting Urukhai moved from Pierce to Ziliang
Kearny PDZ: The Dioscuri stayed on Vackisujfalu
New Syrtis PDZ: New Syrtis CMM stayed on New Syrtis, 6th Syrtis Fusiliers moved from New Srytis to Weatogue, 1st Capellan Dragoons moved from Wernke to Kittery, Lexington Combat Group moved from Wappingers to Monhegan
Ridgebrook PDZ: Ridgebrook CMM stayed oN Ridgebrook, 8th Syrtis Fusiliers stayed on Avigait
Sirdar PDZ: Sirdar CMM stayed on Sirdar, Davion Assault Guards moved from Frazer to Kittery, 39th Avalon Hussars moved from Aucara to Kittery, 15th Deneb Light Cavalry moved from Verlo to Kittery, 3rd Ceti Hussars moved from Immenstadt to Manapire, 1st FedSuns Armoured Cavalry stayed on Ziliang, Illician Lancers moved from Jonzac to Ziliang, Wylie's Coyotes stayed on Bromhead
Warren PDZ: Warren CMM stayed on Warren, 2nd Albion Training Cadre stayed on Enchi
So, there weren't that many units particularly close to the Concordat at the start of the Fourth Succession War; regular units largely got pulled into Ziliang and Kittery, two worlds at the extreme ends of salients into the Capellan Confederation, while other units like the March Militia basically stayed where they were. The troop movements suggest the FedSuns wanted to keep pressure on the Capellan border or tie down defending forces, but they didn't go on the offensive, and of those units that were in sytems brodering the Concordat, like the Coyotes on Bromhead, stayed where they were.
Alternatively, you can take the view that the Taurian border was basically being guarded throughout the war by a handful of March Militia formations, the 2nd Albion Training Cadre, the Dioscuri and Wylie's Coyotes, which I guess says something about how big a threat the AFFS viewed the TDF as being at the time.