My immediate goals are to learn the software and satisfy employer qualifications. I can't afford a lot and don't want to pay monthly, and that is why I am considering an older version.
The Photoshop Photography Plan is actually a good deal. Ten Dollars a month, and you can cancel any time (see here). CS4 is no longer offered by Adobe, so you would likely need to buy a used copy. Be careful; there are lots of copies offered on eBay that contain malware. Assuming you have a trustworthy seller, you would need to properly transfer the license (see details here).
Photoshop = corel photopaint
Illustrator = corel draw
That is an over simplification but should get the point across. There are differences, but the basics theory is the same.
You might find an older version on amazon, but like Semaphoric stated buyer beware with older versions. No matter the source, it is possible to get a bad apple seller.