Prroblem switching user

I run Debian. But I got instructions on fixong it. I was instructed to chmod
660 /dev/ttyS4
Unfortunately, when I went to test it, adter I put login name and password and
then pressed enterthe screen went blank, blacked out, and then the kugin
dcreen appeared again. Then when I tried to login to that account from root
it replied:
bash-2.05b$ su bmike1
Password:
bash: /dev/null: Permission denied
and repeated thr permiddion denied until cntrl-c wasd pressed. What's up with
this?