The card states "Issue an order to 4 units to open fire. Units in a firefight may not be adjacent to an enemy unit, and may not move". I'm assuming this is in reference to infantry? If it were to include tanks and artillery then wouldn't the card read "Issue an order to ANY 4 units to open fire".
"any unit" really isn't used on any command cards giving orders. in most cases, there is some stipulation (such as left section, armor units, or in this case, units not engaged in cloce combat) so "any" would be incorrect. (At quick glance, the only card that uses the words "any unit" is Barrage - which truly allows you to engage any enemy unit on the field regardless of their status).