One does not get permanently locked out of a terminal when using up all guesses; one simply gets a 10-second lock out. As a result, the fourth rank actually just removes the 10-second time out (the password that must be guessed is randomized again).
Terminals appear approximately 70% less than locks in Fallout 4. They are mostly used for activating robots rather than loot.
Additionally, the Hacker perk unlocks the ability to build crude, primitive terminals at settlements in workshop mode. In spite of their smaller, circular screens, these terminals can be used to load any Holotapes the Sole Survivor has in their inventory, and also connect advanced electrical items such as lights and speakers to provide new controls and options.
Locked terminals in Fallout 76 require a hacking skill ranging from 0 to 3 to be hackable. The Hacker perk when used alone allows player characters to attempt hacking terminals with level 1 skill locks, as well as reduces the lock-out time when a hacking attempt is failed. When equipped at the same time as one of the other two hacking perks, Expert Hacker and Master Hacker, player characters can attempt hacking level 2 locks. Player characters that have all three hacker perks equipped at the same time can attempt level 3 locks. Duplicate perk cards cannot be equipped to increase a player character's hacking skill level.