[tomaytotomato]

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)

current would stay the same at 0.2mah

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

[barbegal]

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

[agilob]

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

[Evidlo]

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.