What is the main difference between the AC4490 and the AC4790?

The AC4790 and AC4490 are identical hardware modules, they only differ in the firmware that is loaded to them. The AC4490 has a server / client architecture while the AC4790 has a masterless architecture. The AC4490 requires a server in each network that is used to keep all the clients in sync; all radios hop together through the frequency bins. The AC4790 does not require a server to keep sync and therefore all radios are hopping independently through the frequency bins and only synchronize when there is data to be transmitted.  A masterless radio can be used to build a mesh-type network where a server / client is limited to a star network.