It sounds like your bass was set up with proper gauge strings for BEAD tuning. These would be the same gauge as is used for the BEAD on a 5 string so with a 5 string you'll have the same thing.
Trying to use smaller strings will give very floppy strings with a host of problems....frett buzz, easy to pull out of tune, hard to set intonation, hitting PUs...
Work at it and it'll feel natural before you know it. Get a tennis ball or rubber ball and work up your left hand finger strength. Won't take long.
mech