You can definitely file a flight plan later than that. 30 minutes before departure is the recommended filing time (per AIM 5-1-6), but you can even file for an immediate departure. It's just that at less then 30 minutes additional delay is possible, depending on backend handling and human processing. But in most cases the strip will print within a minute or two of submitting the flight plan.

But also, this is about when the flight plans expire out of the system if they're not activated for departure - so it's saying unused flight plans won't be deleted for 4 hours, so pilots won't have to refile for departure.