So you currently have the three units of light foot described as Tusk Town Guard. Lt Lincoln is responsible for day-to-day management, you get to inspect them and check their behaviour etc. Currently they are based in three watchtowers, that are set at each of the gates through the Tusk city wall.
If a fourth district and watch tower are added, you get a fourth unit.
Technically, Lt Lincoln should be a Lt-Commander, rather than a plain Lieutenant, as he is in command of a reasonably sized garrison.
http://rp.baileymail.net/doku....lines:military_ranks