#You only have one pool of slots
You cast whatever spells you can cast, in whatever manner you cast them. They use slots, which are a generic resource independent of your specific spellcasting class. There aren't 'Wizard' or 'Cleric' slots, for example-- just 'spell slots'. So your PC might cast only Eldritch Knight spells with all their slots or only Wizards spells with all their slots or some combination of both.
#You can't cast firebolt with a 4th level slot
Firebolt is a cantrip. Cantrips don't use slots, and don't benefit from upcasting so there'd be little reason to do so anyways.
But, yes, You should ignore the class tables and just look at the table for multiclass spellcasters.