I'm planing on playing a monk in our next campaign as a backup-tank/semi-tank.
At 1st level he has an AC of 14 without armor with 15 Dex/Wis and unarmored defence.
Is there a better way to increase AC than skipping a feat at level 4 and going straight for the ability increase?
Taking a feat like linguist would be much better for RP but the extra two AC could make a big difference in each fight.
Straight Ability increases are your hands-down best way to increase AC as a monk
Your Martial Arts class feature depends on being unarmored and Unarmored Movement only works if you are unarmored as well.
You start out with no armor proficiencies so you would need to invest a feat to wear light armor anyway, whereas by increasing both Dex and Wis by 1 when you hit level 4 you will also gain the added benefits of adding +1 to initiative, Dex and Wis saves, skills that are based of Dex and Wis as well as increasing the DC of your Ki based spells/attacks by +1 and your to-hit and damage on melee attacks (assuming you are using a monk weapon) both by +1.
Due to monks having a lot of class features tied to Dex and Wis its almost always in your best interest to increase those stats rather than to take a feat.