Pressure fed engines of PSLV's fourth stage (PS4) are hypergolic (MMH/N2O4) and their combustion chamber is regeneratively cooled with their Columbium alloy nozzle being cooled radiatively. They are modified versions of PSLV first stage (PS1) Roll Control Thrusters.

Here is a good view of cooling channels machined on combustion chamber walls.

Image sources:

https://web.archive.org/web/20210101224749/http://asaco.in/aerospace/precision-cnc-machined-omponents/

https://web.archive.org/web/20210101224745/http://asaco.in/aerospace/sub-systems-for-aerospace/

https://www.isro.gov.in/sites/default/files/flipping_book/9-Space_India-234_1989/files/assets/basic-html/page-7.html

https://www.isro.gov.in/pslv-upper-stage-engines-ps4-successfully-restarted-space

Pressure fed engines of PSLV's fourth stage (PS4) are hypergolic (MON/MMH) and their combustion chamber is regeneratively cooled with their Columbium alloy nozzle being cooled radiatively. They are modified versions of PSLV first stage (PS1) Roll Control Thrusters.

Here is a good view of cooling channels machined on combustion chamber walls.

Image sources:

https://web.archive.org/web/20210101224749/http://asaco.in/aerospace/precision-cnc-machined-omponents/

https://web.archive.org/web/20210101224745/http://asaco.in/aerospace/sub-systems-for-aerospace/

https://www.isro.gov.in/sites/default/files/flipping_book/9-Space_India-234_1989/files/assets/basic-html/page-7.html

https://www.isro.gov.in/pslv-upper-stage-engines-ps4-successfully-restarted-space