Please recorfirm this from a ups expert as well, but as per knowledge :
-- ideally a ups should trip when the battery reaches 13.5 volts (same value for all batteries) .. so for four batteries net voltage comes to 54v in ur case the 3 given readings are somewhat similar in total
-- in load condition ideally 1 volt is dropped as soon as the battery back up start (for 4 btys this comes to 50v ) again the sum is similar
now individually these batteries are taking high and low charge ... probable reasons :
... reduced elecrtolyte or specific gravity (if these are not dry or maint free batteries)
.. defective battery or batteries
... improper wiring
what is strange is that the batteries on the outside are getting more charged then ideal level yet underload these drop 2v each which implies a lesser charge storage capacity
my guess is that its the wiring which is at fault .. u need heavier leads to supply charging current to batteries in the middle ... i think u should rotate the batteries put outside btys in middle and then use a heavy wire or double this wiring .. wait a few days then take reading