I have never played Boarder land. But i did play Fallout 4.
Borderlands has a land with adventure around every turn, a chest when you explore, a legendary from killing bosses. Fallout 4 is walk over there, kill the thing. Good. Walk back. Good. It's very dull in my opinion.
Isn't it obvious? You and 3 other Vault Hunters can play the whole story or just roam around in the wasteland trying to collect 17 million guns. While in fallout 4 you're on your own in an rpg land with "authentic" A.I.
All the mods and vanilla options make each playthrough feel new and exciting