Mutagen slots are on the Character page, which is the same place you spend Ability Points. Overall, there are four slots and the last one will unlock at Level 28.
Mutagens can found on enemies throughout the game and are also crafted through the Alchemy menu. By the time you reach Level 28, you should have a lot more than 4 mutagens. All you need to do is fill each slot with any mutagen and the achievement should pop.