X11 was initially developed on Unix-based systems. Usually Unix systems provide a rich number of tools and utilities to get certain things done. Under OS/2, these tools are not installed, but ports are available which are sometimes functionally equivalent to Unix utilities with the same name, but also differ sometimes in a subtle way. This guide will give you hints if you intend to rebuild the system from scratch under OS/2.
Please also read README.OS2 for end-user information, and set at least the environment variables described there.
At the current time, the most recent version available is X11R6.9 This is a full and unrestricted version which comes with complete source code. 4.0 is a highly experimental release, so many features that might have worked in earlier versions, may now no longer work, or work differently. Be aware that for OS/2, X11R6.9 is considered to be alpha software.