I think the article was saying that they store energy for four to six hours before discharging back to the grid. In other words, due to their cost per kwhr, they are used to peak-shave rather than store power on sunny weekends for use during the week. This iron battery is being touted as so cheap that you can build gigawatt-hours of storage and store energy over days or weeks rather than hours.
That being said, I've heard this song before. I started working with lithium ion batteries in 1995, and since then have tested and evaluated dozens of types of batteries, from advanced lead-acid to lithium sulfur to solid state lithium. And I've seen at least 100 breathless announcements of a new revolutionary battery technology that would solve the energy crisis, store power for weeks, be super cheap, and last for tens of thousands of cycles and tens of years. Aquion was one promising one - a "salt water" battery similar to this one touted for home storage. They actually made it into production before folding, and there are a few of them out there. They don't work well.
There have been dozens of others. The lithium sulfur battery. That company (Ovonics) is out of business now. The molten salt battery. Flow batteries (which work but are big $$$ and have environmental problems.) Various sodium batteries. None have worked as well as existing batteries.
In the 26 years I've been following these things I've seen exactly two that have proven out - silicon anode and solid state batteries for very small amounts of power storage (think motherboard clock batteries.) Maybe solid state will work out one of these days for high power/high energy storage, but I tend to doubt it based on the charge carrier mobility problem. Or maybe one of these new technologies will come along and prove itself. In the meantime, the two mature technologies out there (lithium ion and lithium iron phosphate) work fairly well.