$6,000 / month in daycare for 4 kids? You have a sweet deal my friend. At the daycare in my neighborhood this does not cover even 2 kids : https://www.kidspaceseattle.org/enrollment - click on Tuition link at the bottom and weep.
In my experience the price distribution for nannies and infant/toddler daycares in the same area are largely overlapping bell curves so the decision typically comes down to logistics rather than cost.