If you're just using it for basic tasks, yes, the ipad mini 4 is still a viable ipad and will continue to be for awhile. But keep in mind, it is over two years old at this point already. And ipad mini tech was already behind other ipad tech as it is. So It's that much closer to where future ios updates & apps may slow down your ipad mini or not work at all. I believe that if you can honestly use something productively now, you shouldnt play the waiting game, and go ahead and just buy it....however, theres something to be said for waiting too. Especially if you're concerned about the longevity of your ipad.
I could see Apple releasing a new ipad mini as early as March/Spring and then update the ipad pros in the sumer/fall. Thats kinda what Apple did earlier this year when they released the low cost 9.7" ipad before the latest pros. I really could see apple doing this again in 2018, but releasing an ipad mini update in March/spring before the 3rd gen ipad pros later in the year.
Theres also the posibility of Apple releasing an ipad mini pro in March...and then a radical redesign of the 10.5" & 12.9" ipad pros in the summer/fall. Many rumors are circulating that the 2018 ipad pros will mimic the design elements of the iphone x with face scan ID and go borderless...but without oled screens. If this is the case, then what will the screen sizes be? Will Apple keep the current 10.5" & 12.9" screen sizes but just do away with the bezels? How will that affect the current smart keyboard covers? Or will Apple actually increase the screen sizes to fill the space once used for the bezels? And if so, will Apple then release a bezel-less ipad mini size as well? And have 3 ipad pro sizes?
This would be cool...but there are also rumors of Apple releasing an even BIGGER 6.8" iphone X next year alongside the 5.8" iphone X and a 6.1" bezel-less but LED screened iphone 8s! If so, this news may actually confirm the death of the ipad mini...especially if next years iphones will be able to use the apple pencil.
So many possibilities! It's anyones guess. But there is a consnsus of opinion that the ipad pros will see a radical redesign next year. Wether this includes an ipad mini update is not certain.