I've heard that it's about 4-5 pawns (stronger than a regular knight, but somewhat weaker than a rook)
What is the value of a Nightrider compared to orthodox chess pieces?
But king + nightrider still cannot checkmate a lone king, so nightrider is weaker at least in this aspect.

It's difficult to say because the name "nightrider" has very low creative value. We already have a knight.
What are you trying to say?
Indeed, Nightrider has no mating potetial, unlike Rook. But mating potential only contributes little to the value of a piece; most ed-games in Chess have Pawns in them to provide the mating potential, and it is much more important how well the piece supports or hinders Pawns. There are pieces that are far stronger than a Rook, yet have no matig potential. E.g. a piece that could teleport to every square of the same shade as it is on.
I'm guessing 5 pawns...