Slight modifications:

alias F4 {
if (!$away) {
set %bnick $me
nick $$?=\"Away nick:\"
set %away.reason $iif($editbox($active),$editbox($active),$$?=\"Reason:\")
away %away.reason
ame %away.reason
}
else {
away
nick %bnick
ame Back from $awaymsg
}
}

If you have text in your editbox when you press F4 that will be your away message. If no text is in the editbox, a user input field will popup.