Hey! I work in a level 4 as well--I started in October.
I'd say study the policy & procedures on your NICU 1st & foremost (bc that's what you are legally responsible for).
Then study common diseases in your NICU. Usually they are RDS, IVH, PDA, VSD, PFO, ROP, NEC, thermoreg., Developmental care, etc. (can't think of anything else atm).
Assessments is also really important--i.e., noticing increased WOB & what it looks like, what to do for it/what's causing it (for example, if baby was just weaned from ETT to Biphasic and has increased WOB, maybe they aren't tolerating it & a blood gas needs to be done early. Or if they haven't been weaned, maybe they have secretions that just need suctioned out!)
Good books are 'Merenstein & Garners Handbook of Neonatal Intensive Care'; the 'Core Curriculum for Neonatal Intensive Care Nursing' (by Verklan & walden), this site General | Newborn Nursery | Stanford Medicine for assessments head-to toe; and for review Qs, 'Certification and Core Review for Neonatal Intensive Care Nurses' (by Watson).
I know this response is a little late but hopefully it's helpful!! From one NICU nurse to another :)