Great question… near and dear to my field artillery heart!
Unfortunately there are no specific rules with regard to direct fire. Typically a unit will practice direct fire about once a year. When funding is tight you may not get this practice. A field artillery crew typically switches to direct fire mode when they can visualize the enemy within 2,000 meters but this is dependent upon terrain and other elements. In addition direct fire would be most effective when the gun crew can track and fire at a target faster than Fire Direction Control can provide mission data. With regard to use of the AT4’s these are usually only used to cover choke points and/or against lightly armed or heavy armed vehicles. They are also effective for clearing bunkers or other hides.