First some basic background:
As you level up your character, you unlock skills which can then be assigned to LB, RB, and LB/RB.
In order to buff/increase the effectiveness of these skills, you can equip mods on your 5 pieces of armor (helmet, chest, pants, gloves, boots). You could also theoretically be very lucky and find pieces of armor with 4 mods for the skill you want to enhance.
In order to unlock mods, you must find armor pieces that have mods and then dismantle them, which makes the mods available for crafting when you visit Zahedi at one of your camps. He is available at your camps throughout the game.
In order to unlock this achievement, you need to enhance the same skill with 4 different mods. There are tons of combinations that work, but the one I used was the thermal bomb for the pyromancer. About 10+ hours of playing and dismantling gear, I had 4 different mods for the thermal bomb available. I went to camp, added one mod to 4 different pieces of armor and then used the thermal bomb on the next enemy and the achievement unlocked. I dont believe that hitting an enemy is even required, as another player I was playing co-op with added 4 mods for his turret and used it in the open and his achievement popped as well.