Can we monitor rhel 4, HP unix 11.11 servers in UIM 8.5.1 version. Any workaround if we cannot apply without making the OS upgrade. And let me know any changes can we apply to get that same.
For unsupported OS version the only possible answer is really the RSP probe.
rsp (Remote System Probe) - CA Unified Infrastructure Management Probes - CA Technologies Documentation
This does have LUA scripts that can be modified to accommodate different OS that are not supported by default.
Also you can change the commands used if needed to get CPU, memory and disk information.