@kernal
The AmigaOne has not been made for years. While some owners swear by them, all models have known defects and are prone to a high rate of failure. Despite the known flaws, the price of a second hand board is very high. Unless you have deep pockets and a high risk tolerance I would recommend avoiding these boards.
A second hand AmigaOne is the only legal way to run OS4. If you do end up taking this route, make sure the previous owner has the original OEM CD and is willing to take multiple bizarre steps to legally transfer the ownership of his/her OS4 license.