differs a lot from the left battery to the right? from left to right, each battery uses successivly less water from a lot to hardly any each time I add water. these are the standard wet batteries exactly the same as those used in fork lifts.
If you have batteries in parallel, check the wiring. There is a way to wire them diagonally so that they all have equal wire lengths and participate equally. In the worst case, if the 4 batteries are wired straight across in parallel, the one that is nearest to the load is doing all the work during discharge, and is the one charging up during the day. It doesn't seem like a length of fat wire should make much difference, but it does.
If I understand your question you have 4 total 6 volt batteries constituting a bank of 4 batteries in series/parallel, which should produce a high capacity 12 volt output. First of all I'm assuming that all of these batteries are in the same circuit and would be subject to the same loading and charging and they are wired correctly. That being the case I would be surprised if there was a great difference between any of the four batteries use of water. It sounds to me like if some of the same batteries are requiring lots of water at whatever interval you are checking there is a problem, especially if one or more of the others aren't. If this is the case my advice would be to contact the battery manufacturer and find out the proper process to equalize all four batteries. If you have one or more of these batteries using excessive water there is a problem. Remember this principal: When using two or more batteries your power is only as good as the weakest battery. That is why you always replace flashlight batteries together with two new ones. It sounds as though one or more of the batteries are sulphating, which will greatly reduce their capacity and generate excess heat boiling off the electrolyte prematurely. Check out my suggestion to equalize the batteries, which if they aren't too far gone will get them all back on equal footing again. Good luck.
