That's the common thing that people struggle with moving from a 4 to a 5. I know I did, but I'm teaching someone who's recently moved to 5 or 6. I stop them playing anything below the 5th fret, apart from the low B notes , they're banned from playing below the 5th, at least in my presence .
They're playing a 5 or a 6 string, but still 'thinking' 4 string, the thought is all about the new low notes, but there's a whole lot more to it than that.
I find it just helps to forget the 4 string thing, and to move on and adapt to the extra string(s).