Hello
It's also depend on "Portability Tools", such as Bristol, MKS, QT, etc. If your program is written for several platform using the portability tools, and the tool is already ported to AmigaOS 4, then "in theory" it will be easy to port the application to AmigaOS 4.
Using AmigaDE, the same binary program will run on top of Linux, Windows, Windows CE, Symbian, Qnx, and (in the future) Amiga OS 4.2 etc.
