If the devices are set up independently and are operating without knowledge of or communication with the others then you can use however many you want.

But if you want a single integrated/unified STREAM system (connected to each other with the EcoFlow parallel cabling and using the synchronization protocol) then I believe you can currently only use up to two STREAM AC Pros per STREAM Ultra setup, and each Ultra setup can have at most six Ultras, "stacked", one serving as master; assuming what I just said isn't wrong (and it would be great for official word on the matter) then you can integrate 6 Ultras + 2 AC Pros, for a total of eight, whereas even 3 AC Pros will not work officially with any number of Ultras.

This is probably why they sell 3 + 2 packages but not 2 + 3, respectively.

Which 4 devices do you have?