Hello Sean ..,
Just my opinion, but, if you have four batteries in PARALLEL, and you are getting a significantly different reading when using your DMM to measure between the two TERMINALS on one battery, then there is a very poor connection somewhere. Make sure that you are measuring the battery terminals.
This type of situation is one of the reasons, that parallel battery strings are so problematic for almost all who try to use them.
Clean all battery terminals, and lugs on the battery cables, carefully. Measure the voltage of each battery, when interconnects are not connected. Use petroleum jelly on all of these contact surfaces, and reassemble.