Just (potential energy) * (efficiency)

I am going to assume 100% efficiency for simplicity. [1]

mgh Joule = 20 Watt * 4 hour * 3600 s/hour

(Joule = kg * m^2 / s^2 = W * s)

Lets say you pump water to a height of 10 meters.

m = 20 * 4 * 3600 / (9.8 * 10)

m = 2938.77 ~= 3000 kg

So you need about 3000 liters (1L = 1kg for pure water) of water at a 10m high tank to power a 20W bulb for 4 hours. This about 800 US gallons. Considering other inefficiencies, you'll need more.

[1] http://www.jcmiras.net/jcm/item/93/ (first google result for efficiency of a generator) says typical efficiency of a water generator is 0.95.