Unfortunately, The Sims 4 doesn't allow for hybrid Sims at this moment in time, though we'll update you if they do! To become a Spellcaster, your Sim will need to give up any other life state it may have before it can transform. It all depends on which types of magic they're learning and there's no need to stick to one, either - your Sim can be under all four at once if they so wish. Spellcasters can be a part of four Schools of Magic: Practical Magic, Mischief Magic, Untamed Magic, and Alchemy. This is essentially their Mana, with the bar increasing the more they use magic. They will also gain the Spellcaster Charge meter. Once your Sim becomes a Spellcaster, they'll gain access to their own Spellbook, which includes Perks and castable Spells, and a Glimmerstone which lets them fast travel back to The Magic Realm from anywhere without needing to use the Mystical Portal. Unlike other life states, it has little effect on appearance so they'll still look human bar some cool eye color options and optional outfits. It is a magic class and focuses on casting spells, brewing potions, conjuring Familiars, and inflicting Curses. A Spellcaster is a new life state, like Vampires and Mermaids.