I think it's a great upgrade today and (unfortunately) will probably be a great upgrade for another year or more. And, honestly, if Apple surprises us, you can either hold off upgrading to the Mini 5 until you feel like you've gotten the value you want out of the Mini 4, or sell it and upgrade. Might it cost you a hundred or two? Yeah, I guess so. To me, the Mini 4 was such an upgrade that I'd be happy if it worked out that way. Hope it works out well for you and you enjoy the new Mini.
I don't plan to upgrade to IOS 12 & 13.