In electrical terms 4 GJ is 1,111 kWh. That's about 15 EV batteries' worth, or about £220 worth of retail electricity (at 20p/kWh). So it's a lot compared to the usual domestic things people are used to, but not much in grid terms.

Or if you consider the Irish grid (average consumption around 5 GW) that's enough energy to power the grid for about 0.8 seconds (obviously it's not going to have enough instantaneous power output to do that, but again for a sense of scale).

If Ireland had 10 of them, that'd be 8 grid-seconds worth of energy. Although, of course, actual disturbances aren't going to be that large. A few percent imbalance perhaps?

So if the whole grid had an instantaneous 10% imbalance, one of those units could carry it for 8 seconds.

(EDIT: changed energy numbers to fit the appropriate power grid)