Setting headers using the /bin/mail command

There are many different versions of mail (see Mail vs. mail what is the difference and the Heirloom project's write-up on the different versions of mail).

If you want to keep your sanity, I recommend avoiding any tool whose name is too close to mail. Mutt is a lean text mode mail client which is often available, very flexible, and behaves the same everywhere.

mutt -H - "$2" <<EOF
From: $1
To: $2
Subject: $3
Importance: high

$4
EOF