A lot depends on how low , below the top of the plates the level was and how heavily they were charged while low; possible deformed plates due to heat. Slow charge for 48 hrs then disconnect, from charger and loads, for 24 hrs and test voltages.
6volt in good condition should test at 6.3 v approx. If one checks out bad , below 6v, replace all 4 .
If they are 12v batteries they should check in at 12.4 to 12.5 under 12 replace.