Yes, most fights (equal CR ones) are supposed to be 4 on 1 cakewalks for PCs.
And yes, that means that most characters are not living up to their CR.
CR is wonky as hell, though. Eyeballing is generally a better idea. Get a good idea of your party's capabilities, and plot out the fight - if your guy goes down too fast, make him tougher. If he slaughters the party super fast, lower his offensive potential or water down his schtick.
Generally you want to have a guy that is hard to kill but not necessarily going to instantly wipe out the party. This is fine, because most real-life combat disciplines focus on defense because getting killed is worse than not killing the other guy. The opposite, all-out-attack, do exist, and you can have that as a boss, but it means the party has to desperately hold this guy off long enough to kill him, and you'll have a higher chance of death, defeat, or TPK.
Typically ensemble bosses (boss + terrain + minions) are more interesting and better fights than just one monster in a room.