Very little resemblance to FP4. A unique film. I shot a lot of R25 in 120 format, mostly in the high mountains. It had a distinctly longer scale than FP4 in terms of handling scene contrast, but was more finicky about exposure. It was orthopan sensitive instead of pan like FP4, and distinctly finer grained, but with excellent acutance. And it was obviously slower speed. It was around the same real-world speed as Pan F, but had a very long usable straight line portion, whereas Pan F has just about the most exaggerated S-curve as any current film I can think of. And it's a true general-purpose film, and not something needing to be bullied into questionable tonality like Tech Pan or other micro films.
A remarkable product, but with some cons : the antihalation properties weren't as good as most 120 films, so you had to be extra careful to handle and load and unload the camera in the shade. The emulsion was quite sensitive to too strong a stop bath; it needed to be quite dilute. And the last batch of it had a lot of contamination with emulsion grit and so forth, as the factory was winding down and no longer kept up.
I developed it in PMK pyro, and got some remarkable prints from difficult extreme lighting. Wish it were still around. Ive been told it doesn't scan all that well. But I couldn't care less about that. Real darkrooms make scanning obsolete.