Welcome to amiga.org!
If you want to use AmigaOS 4 you will have to find yourself a used AmigaOne system (I saw one for sale on amigaworld.net earlier today), since there is no new hardware available at the moment which will run AmigaOS 4.
There are some new systems being developed which MIGHT be able to run it, but don't hold your breath, they might as well never be released at all...
