As per the FAQ, all ports should have antenna connected.
"On a 4x4:4 AP however such as the MR84 and MR86, there should be no unused ports. These APs have four dual-band antenna ports (and therefore do not have 2.4 or 5GHz labels). None of these ports should be left unconnected because all four ports will always be active even if one radio is disabled. MR84/86 APs should have all four antenna ports connected"