I have installed RHEL4 recently,created new user feapadmin. feapadmin is a member of group feap,bin. most commands avaibale in /usr/bin are giving same error permission denied when I loggd in as feapadmin,it works fine in root. to backup to tap, iused comand tar cvf /dev/st0 /usr2
but got the same error permission denied. also tried crontab -l but displyaing msg. permission denied.(Note that I have desabled SELINUX). firewall is also turnd off. awaiting good solution. thx.

I would implement sudo. Sudo (su "do") allows a system administrator to delegate authority to give certain users (or groups of users) the ability to run some (or all) command. Using sudo you can run the backup command as feapadmin with root privileges.

See http://en.wikipedia.org/wiki/Sudo for more info.

RHEL4 doesn't require sudo to do crontab for users and most commands. (Of course the crontab updated will be the user's crontab rather than root's.)

Was SELINUX disabled during install or afterwards? If afterwards you have to reboot to make it take effect.

The fact you're getting permission denied suggests there is an issue with permissions on /usr/bin or the executables you're trying to execute there.
As root do ls -ld /usr /usr/bin /usr/bin/crontab and verify permissions are read on the first two for all users and read and execute on the last one for all users.

Add the username to trusted in /etc/group.