From what I gathered from the info provided, the water pipes in each unit are bonded, they are not ground electrodes. I am assuming that the underground portion of the water services is plastic, if not the following does not apply. The code requires a continuous conductor to the main ground electrode, in this case the ground rods. The H2O bond wire is allowed to be tapped/spliced as long as it is sized to the service entrance conductors, not the breaker size. I don't know what size SE conductors were run, but fromwhat I see this is a code compliant installation.
Now after rereading the original post, if the water underground services are indeed metalic... you would have to go to the main ground electrode unspliced. Could any ONE of the water services be the main ground electrode and the other 3 be supplental?
I think that is how I would look at it in my town.
If this is the case as long as you go to any one of the water services unspliced (even the last one) you could splice to the other 3.