Tony,
Thanks for the question.
I haven’t recorded a video for this lesson yet but I likely will at some point.
Sure, it’s totally fine to use your third and fourth finger to fret the 5th string if that feels easier.
I like to use my thumb a lot of times to fret the 5th string because I have a slightly wider neck on my banjo and it’s a little easier for me.
But it’s personal preference so feel free to do whatever is most comfortable for your hand.
– Mike