Remember that very few devices are controllable in RF. RF control from a universal remote like Harmony involves transmitting an RF signal to an RF receiver (the "hub" that twizt3dkitty mentions) which then generates IR signals to the devices, so it's really IR control. The Directv boxes ARE controllable in RF but there is no universal remote that will directly control four of them.
And unless the TVs are all from different manufacturers some of them will respond to the same IR codes.
The only technology around that might work for this is some sort of IP control, assuming all the devices are internet-connected, but again I don't know of anything specific, some of the high-end remote systems might allow you to this.