Is it okay to play a note on 4th fret with ring finger?

I've learned from the beginner tutorials that the first 4 frets are allocated to point, middle, ring and little fingers respectively.

But recently, in a guitar lesson for a song, I've seen the guy sliding the fingers and used his ring finger for the 4th fret to play a note. Is it a correct approach?

Even if it's hard, I want to follow the correct method. Which finger you want me to use to play a note on the 4th fret? The ring finger or the little finger?

You should use whatever is more effective, comfortable, economical, etc.

Some traditional methods of guitar will insist on using certain fingers for certain strings or related ideas like hitting repeated notes on one string by rotating through index-middle-ring, etc. There is often a good reason for this, but none of these work in all situations — much less are always the best idea. Take them as guidelines.

However, you do want to make use of your pinky on songs where it's not "needed", at least when practising, because it is generally the weakest finger and you want to build it up before you do need to rely on it.