Originally Posted by
rocknrollin
Officially they are not compatible with Shimano STI's, but I've seen 2 independent blog posts of people pairing them with the Ultegra STI and being satisfied. The larger body and finned pads should result in a higher heat tolerance.
A little of the topic, of the RX4, but definitely related...
I have a gravel bike that has a post mount fork/frame, so newer flat mounts were out fo the question. It had the 785 calipers, but they've been problematic for more than a year, leaking, and even after rebuild, just seemed so much capable than my XT series brakes on my XC rig. So, I first tried to source post mount RX4, and I searched for a couple of weeks, and finally gave up 2 weeks ago and ordered the XT 8120 (4-pot) and XT 8100 (2-pot) for my gravel rig, and paired them to GRX brifters, and they work just fine, in fact I'd qualify them as "great." If you look
here, Shimano lays out every lever that uses their "ServoWave" hydraulic action (pad engagement earlier in travel with a larger modulation range until "full lock"). It includes every hydraulic brifter they've ever produced for road/gravel application and all "newer" MTB levers. It makes no distinction between the lever operations, just explains the hydraulic action and implies it is operationally identical across all levers, which then stands rot reason that the calipers are designed to operate identically whether spec'd for a road or MTB group. So, I took an educated chance based on my understanding a bit about hydraulics; even if Shimano doesn't expressly discuss/describe mixing road levers and MTB calipers, knowing that they do not expressly warn (with big red letters, so to speak) against it, I was confident I could do it (and I did seek a few outside azimuth checks to make sure I wasn't off base). So, I did it, and as I said, it works great. I'd also suspect that Shimano doesn't warrant the use of the calipers in this configuration.
That said...4-pot calipers generally are not more powerful than a 2-pot. All things equal, the braking power comes from the lever pushing the fluid into the caliper. Where 4-pots excel, and why they are popular (essentially standard at this point) with downhill MTB racers is the larger longitudinal surface of the pad AND the larger caliper body leading to better resistance to heat fade. I'd expect this with any 4-pot. I haven't read a comparison between the finned Shimano pads and the ones Hope specs, but I suspect that the overall performance difference is not dramatically different.
Long post, sorry. Bottom line, there are more options out there, if they aren't readily apparent win the Google machine.