Maybe I misunderstood how you have your system wired. I thought it was like this:
Positive External Terminal >----
---->+|+(reactor 1, cell 1)-|- (wire connection)>----
---->+|+(reactor 2, cell 1)-|-(wire connection)>----
---->+|+(reactor 3, cell 1)-|-(wire connection)>----
---->+|+(reactor 4, cell 1)-|- >----Negative External Terminal
Positive External Terminal >----
---->+|+(reactor 1, cell 2)-|- (wire connection)>----
---->+|+(reactor 2, cell 2)-|-(wire connection)>----
---->+|+(reactor 3, cell 2)-|-(wire connection)>----
---->+|+(reactor 4, cell 2)-|- >----Negative External Terminal
Positive External Terminal >----
---->+|+(reactor 1, cell 3)-|- (wire connection)>----
---->+|+(reactor 2, cell 3)-|-(wire connection)>----
---->+|+(reactor 3, cell 3)-|-(wire connection)>----
---->+|+(reactor 4, cell 3)-|- >----Negative External Terminal
...and so on for all 20 stacks where the cells are numbered 1 through 20 in each reactor. Even though they are in seperate housings (reactors) and only connected by wire, the cells are still connected in series and considered a stack of four cells. If you check the voltage of an individual cell you will read somewhere between 3 and 3.5 volts (maybe a little more or less, depending on your power source), but maybe I have misunderstood how you have them wired.
What you show above would have each and every cell directly connected, individually, to an external power source terminal and an external ground terminal. If this is the case you couldn't possibly have a more inefficient configuration and will read the full voltage of your power source at every individual cell when you check it.
"Sell your cleverness and purchase bewilderment"