That could be a big potato battery bank?

According to google a 200g potato give off about half a volt (0.5v) and 0.2mah

    4000 tonnes = 4,000,000 kg = 4,000,000,000 g

    num potatoes = 4,000,000,000 / 200 = 20,000,000 potatoes

    volts = 20,000,000 x 0.5v = 10,000,000 volts (10megavolts)

I am not an electrical engineer, what could we do with this?

The energy comes from the metal electrodes not the potato. Potato is just an electrolyte carrying current between the cathode and anode.

Thinking about wattage is more useful. We'd get about 2MW so you could run 20k-ish homes (1kW average across a day) for a short time until the potato energy is depleted.

You'll also need to buy the metal electrodes.

I think it might better to produce alcohols from it and then burn the liquids and gasses.