On my main PC I have every settlement established. They all have at least one clinic, food, and general trade shop. I also have supply lines, and more water and food production than settlers at every settlement. The population seems to cap out in the mid 20's at most of them. I would like to get one of them, preferably Sanctuary to 30 so I can utilize the level 4 venders that require that population number.

My Sanctuary is currently sitting at 24 population but I cannot move any more settlers from other settlements. The option is disabled in the move menu for Sanctuary and a few other places. I assume I have hit some sort of limitation that prevents me from moving any more people there. Does anyone have any suggestions on how to increase the population other than moving settlers?

FYI my Sanctuary currently has 33 beds, 40 food production, 80 water production, 150 defense, and every level 3 shop.

If I remember correctly, get your Charisma to 20 to get 30 Settlers in a Settlement. However, the Level 4 Vendors require a maximum of 30 Settlers for all connected settlements not just one settlement. So if you have 24 Settlers at Sanctuary and 10 Settlers in another Settlement, then using one of the Sanctuary Settlers to set up a Supply Route will get over the required population number.

Settlement population = 10+charisma, which maxes out at 11.

Also, you dont need 30+ people to use the level 4 merchants, i have no idea where you got that idea from. All level 4 merchants can be found in the game world, and once hired, and be used at any settlement regardless of the population there.

It is from http://fallout.wikia.com/wiki/Fallout_4_merchants. The Scribe (Level 4 Armor) needs a settlement population of 10, Ron Staples (Food&Drink) and Doc Anderson (Clinic) needs a settlement population of 20, and Smiling Larry (Weapons) needs a settlement population of 30 for that wiki page. Apocalypse Armor is sold by the Scribe and Vault-tec Representative and Reckoning is sold by Smiling Larry.