Beware the link you posted (iDevelopement.info) is 7 years old and shows how to install an obsolete update of Solaris 10 on a 11 years old hardware. That's probably not the best starting point and is actually not advisable.
Your T4-2 server provides two way to achieve what you want without reinstalling the OS.
You can create a Solaris 10 branded zone from a template made on an existing server running Solaris 10 and install the application requiring it inside. You might not have that server available so there is another virtualization technology with which you can install Solaris 10 without touching the existing OS: Oracle VM Server for SPARC (a.k.a ldoms)
Both of these will require root's privileges but you might not need root password. By default, root is no more a user on Solaris 11. What you need is to know the password of a regular account allowed to have root's privileges, either configured with sudo or rbac. If you are unable to gain root access that way, you still have the possibility to boot on a Solaris 11 DVD. Select the "user shell" choice, it should recognize the existing file system and ask you about mounting it. Accept then simply edit its /etc/shadow file, put the hash you want (or clear it) there and reboot.