Bass tabs on the internet can be kind of confusing and don’t always follow the same format. My guess for this one is Repeat? Perhaps you can listen to the song and figure it out? If there are a lot of repetative notes on the fourth fret of the e-string, then perhaps the author meant 4–4-4–4-4–4-4–4-4–4-4–4

Try “rest”, it’s my best guess but it makes sense to me. But that is only a SWaG.
I read sheet music and never got into tabs.

It could be for a tap. I have seen taps marked as a T and also sometimes a R or L indicating which hand to tap with (and I believe all tabs are written assuming you are right handed).