Some of the more obvious differences between SunOS 4.x releases and the SunOS 5.x releases are:
Many UNIX commands have changed. For a detailed list, see Solaris 1.x to 2.x Transition Guide.
The printing subsystem has changed. For example:
The mail programs have changed. The SunOS 4.x mailboxes and folders are completely compatible with any of the three Solaris 2.3 mail programs:
The location or name of some system files has changed. For example:
For more information on the preceding topics, see Solaris 1.x to 2.x Transition Guide.