The iPad 4 is three generations and about four years older than the Pro. An iPad 4 probably isn't a good investment at this point unless you're getting it REALLY cheap.
The iPad Pro has keyboard and Apple Pencil support, while the iPad 4 doesn't. There is also updated internals in the Pro, as well as upgraded storage options.
As mentioned before, go for the pro if money isn't a problem, or go for the 4 for a good price. You may want to consider the Air 2, as you can get those for a decent price and should have a longer useful life than a 4.
The major differences is the iPad Pro (9.7) has Tru-Tone display, Apple Pencil, quad speakers. It's approaching it's one year anniversary in March. Outstanding iPad.
Maybe....Mini 4 is awesome! It fits in your purse. Easy to hold it with one pretty hand. Very PORTIABILY!! iPad Pro depends on what size you are looki g for. Bigger 12.9 screen is nice for watching movies or "artist dream" but you will need two strong hands to hold forever. Also it lacks professional apps. (Microsoft Office app is not free). 9.7 iPad Pro is for "everything" stuff with a better TrueTone screen (12.9 iPP does not). You can either hold it with one or two hands. Microsoft Office app is FREE! Both Pro tablets come with 4 awesome stereo speakers. (Sorry I can't hear because I'm deaf).
If you can wait until next month for Apple's biggest events for new iPad line. You never know what you are missing with new iPad Pro.
For me I just bought a 12.9 iPad Pro few months ago because I had enough with 9.7 iPad for several years.