I guess the point is that when combined with a source of dispatchable base load power like a combined cycle gas turbine or nuclear, batteries are now cheaper than open cycle gas peaker plants for bridging short (4 hours or less) spikes in demand.
This is exactly what they are going for. To not have to build and run expensive gas plants just for a few hours in the evening. It also allows you to run the few gas plants that you do build for longer to better amortize their build costs, making them less expensive overall.
Ideally you would pair these 4-hour batteries with CHEAP sources of electricity. Fill the battery when electricity is cheap, empty the battery when electricity is expensive (ie, when demand spikes or when other sources of generation drop off the grid).
The cheapest source of electricity today is noontime solar. Batteries can fill up on noontime solar and discharge into the evening peak. They could also charge on wind or traditional baseload sources overnight, and discharge into the morning peak.
