no, it's a valid and comparative way to scale their power. None of the main PCs' companions are as strong or are ever depicted to be as strong as the PCs themselves.
And it's not just any droid but the infamous jedi-killing assassin droid of Revan's design, who accompanied Kreia and the Exile and definitely was no pushover.
I never said anything about companions, the 4 PC's confronted HK-47 yes...but to say all 4 of them were needed to kill him? No. It doesn't matter that HK-47 is a jedi killing assassin droid, he is still a droid and besides his track record hasn't been steller thus far. Infact, what Jedi has he killed anyway that is proof, hard proof names and all? He has knowledge of hunting them yes, but any droid can be downloaded with information and spew it out like a computer.