Pressure fed engines of PSLV's fourth stage (PS4) are hypergolic (MON/MMH) and their combustion chamber is [regeneratively cooled][1] with their Columbium alloy nozzle being cooled radiatively. They are modified versions of PSLV first stage (PS1) Roll Control Thrusters.

[![enter image description here][2]][2][![enter image description here][3]][3][![enter image description here][4]][4]

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

[![enter image description here][5]][5]


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


  [1]: http://iafastro.directory/iac/archive/browse/IAC-16/D2/1/35442/
  [2]: https://i.sstatic.net/00l4fm.jpg
  [3]: https://i.sstatic.net/YLZv5m.jpg
  [4]: https://i.sstatic.net/jgXYkm.png
  [5]: https://i.sstatic.net/N2cZE.jpg