I don't think you'll find an oil paint filter for photoshop cs4, since it wasn't until photoshop cs5 and pixel bender that had the oil paint filer and as far as i know, adobe
has never made the oil paint code public.
There used to be lots of pixel bender filters for photoshop cs4 on the adobe exchange, but adobe shut it down.
https://forums.adobe.com/thread/1513504
You can look at what was available and see if any external links are given to the authors websites where you still be able to get some of them.