Paint Shop Pro is a very good program.
It is not as nice as Photoshop (any version ) to work with..........the GUI is comparatively ugly and clunky and the whole thing is not so intuitive as Photoshop.
It is a long time since I had Paint Shop Pro, and that was in the days when it was a JASC product..........I believe that it is nicer to work with now that it is a Corel product.
I think the question may be...........what more will you get from Photoshop CS4 than you have with Paint Shop Pro. Mainly the ability to prepare images to be sent to professional or industrial printing presses......a bit of overkill if you are going to tweaking and repairing your Dad's photos for home printing.
You may just find that Paint Shop Pro does the job just fine ( which it will ) and that the extra bells and whistles in Photoshop CS4 are superfluous to requirements.
Don't be dazzled by the name Photoshop, it is very, very good but the pro versions such as CS4 are really not intended for just twiddling with your photos.........it is a huge creative tool aimed at pro output...and you pay for that, and pay through the nose in this country.
£25 is a good price for Paint Shop...........is that for a new, unused copy ? If second hand, read up on the requirements for validating etc
If your Paint Shop is a new, unused copy........go for it.............if it is secondhand....be careful