Below is the iOS compatibility matrix:
http://iossupportmatrix.com
Based on this, looking at when Apple stops providing OS updates to old chips, and since the Mini 4 has an A8 chip, I expect it to support iOS 12 (not sure if that will be done well) but not iOS 13, that would appear in about 21 months. Beyond this, you may not be able to update the OS, hence to install or update many Apps. If you need some Apps (and are not using only the built-in Apps + those you will have installed already), the use of the device may become frustrating beyond this date. So the question is, is this enough use for you ?
(I am not entering the discussion of the format ---I personally love the format of the Mini, but would need it to support the pen)