As of now, you havent much choice since amigaone boards aint avaliable from anywhere but second hand. The AmigaONE is sadly at the moment the only system that runs OS4. The pegasos will probably never run OS4, but runs a OS called MorphOS which has built in emulation for running old os3.x 68k apps.
There are talks about a new OS4 compitable board coming out sometime in the future, which has also pretty much been verified by hyperion which those who are coding OS4. Their website is:
http://www.troikang.com/I personally would wait and see if something comes out of this, unless you can find someone willing to sell you their amigaone.
Check out these forums: For OS4/AmigaONE
http://www.amigaworld.net/ and for MorphOS/Pegasos:
http://www.morphzone.org/
