When it comes to lead guitar playing, many players have been able to create effective styles using only their first three fingers.
However, even a player like Michael Schenker—who is renowned for his prowess with three fingers—uses his pinky sometimes. There are certain licks that are just easier to play if you use your little finger.
It allows you more reach and it doesn’t require you to extend the distance between your second and third finger joints too far.
However, if you’re mainly a pentatonic player who uses a lot of two-note-per string patterns and doesn't use sequential three-note-per-string patterns, then you could very well do everything you need with your first three digits.
It comes down to what you want to play and what you need in order to play it.
It may also be the case that you’re comfortable using your pinky, except when you move to the higher frets. If the frets are closer together, and using your pinky is too cumbersome, then reverting to using only your first three fingers isn't a bad choice.
Players like Yngwie Malmsteen use their first three fingers for certain shapes only, while using their pinkies for wider modal shapes.
So the answer to the pinky question is that you don’t have to use it per se, but it certainly won’t hurt your playing if you do.