
People would love a 5 Mystic, but I wouldn't be surprised if the Discipline is the way to go for her at the start.
EotE investigators all have the same deckbuilding tension: you have 5 slots to devote to Level 0 in your second class, but if you upgrade out or otherwise replace them, you don't get to backfill those L0 cards for free. So if you want a Survival Knife later on, it can feel wasteful to put the L0 version in your deck at the start - even if it might be helpful in that first scenario. Same goes for fighting staples like Beat Cop and Vicious Blow. Between working around that, and her refusal to pick up a gun, Lily has a hard time leveraging the 5 out the gate.
4, though, opens up some options. She can get that to 5 or 6 easily between Holy Rosary for soak, and David Renfield for soak and blood money, fueling a powerful suite of Mystic events like Spectral Razor, Read the Signs and Storm of Spirits, many of which end up replicating the effects of L0 Guardian cards, but ... better. More importantly, with 4-6, spells like Azure Flame basically become your guns - except unlike guns, they're just as good at blowing nasties out of the sky as they are putting ghosts to rest. Having that much gives you a resiliency many guardians would envy, and it helps that the associated Discipline offers some built-in healing, while being fairly easy to mend when broken.
Using spells-as-guns lets you run enough "weapons" to lean into the "I'll draw something, probably" school of deckbuilding so you aren't using your L0 Guardian slots on tutors like Tetsuo Mori and Prepared for the Worst like many Guardians do for consistency. This lets you run other L0 cards that don't have higher-level equivalents, like Steadfast, which is good for everything you do, or support cards like First Watch - cards that can keep paying off to the end of the campaign. And once you start entering the L1-5 Guardian territory, you can keep leveraging that with cards like Enchanted Armor and Enchant Weapon.
Also - Delve as deep as you want. You've got stats on the line, and if anyone complains, tell them you're the chosen one.