Buy for Better Hardware
The iPad Mini 4 launched back in 2015 which means its hardware is now several years old. The hardware (cameras, processor, etc) is still good enough for some people, but others will benefit from the iPad mini 5's upgrades.
The iPad Mini 4 uses an Apple A8 chip with an M8 motion co-processor. This is old technology compared to what's on board the iPad mini 5.
The iPad mini 5 features a new processor (A12 Bionic chip) that Apple says helps the device deliver "three times the performance and nine times faster graphics." That's huge on a tablet like the iPad Mini that's meant to be portable.
The iPad mini 5 also features an advanced Retina display with True Tone technology and wide color support. Apple says the display is 25 percent brighter than the iPad mini 4's display and has the highest pixel density of any iPad in its lineup.
The iPad Mini 5 isn't a workhorse/potential laptop replacement like the iPad Pro, but it will get the job done for a lot of people.