What about anti-submarine operations? Non-carrier warships could carry helicopters instead of floatplanes - and helicopters should be a lot easier to launch and especially recover than floatplanes.
They´ll probably lack the range and speed to perform naval search missions, but for ASW, and perhaps artillery spotting, they should be useful enough.
Looking at the R-4 and H-6 it looks like the options are:
Pilot + 1 small depth charge
OR
Pilot + passenger with binoculars
Assuming there is already a small radio fitted then (extremely basic) ASW and spotting are certainly feasible but OTL they arrived when the allies had decent numbers of MACs carrying g real ASW aircraft and plenty of conventional Austers, L-5s and whatnot for spotting.
Move them forward a few years and a I could see convoys defended by a few CAMs each with a hurricane on the bow and an R-4 spotted on the stern.