Fluorescent lamp issues
These fixtures are usually wired so that the (2) outside bulbs and the (2) inside bulbs are wired together. Yes both bulbs need to be inserted in the sockets to light-up. Also the 4 foot bulbs with (2) terminal pins need to be turned so that they lock in place and contact the terminals carrying the voltage. Usually a 1/4 turn or until resistance to turning is felt.