4000 is setuid "set user id on execution"
from "man chmod"
"SETUID AND SETGID BITS
chmod clears the set-group-ID bit of a regular file if the file’s group ID does not
match the user’s effective group ID or one of the user’s supplementary group IDs,
unless the user has appropriate privileges. Additional restrictions may cause the
set-user-ID and set-group-ID bits of MODE or RFILE to be ignored. This behavior
depends on the policy and functionality of the underlying chmod system call. When
in doubt, check the underlying system behavior.
chmod preserves a directory’s set-user-ID and set-group-ID bits unless you explic-
itly specify otherwise. You can set or clear the bits with symbolic modes like u+s
and g-s, and you can set (but not clear) the bits with a numeric mode."
755 is just rwxr-xr-x. standard permissions.