You can use Astropad with the Mini 4. If you want to use a stylus, the Apple Pencil will not work. However, there are several others out there you can use. I used to use the original "Pencil" by Fifty-Three. Adonit & Wacom both also make pressure sensitive styli. It basically comes down to personal preference and your style.
That all being said, if you eventually have the means to get an iPad Pro and Apple Pencil, you'll never use another stylus. The pressure sensitivity and palm rejection are phenominal.