The 4000kg were primary used for making some advertising at the beginning of Air Omega. The airline went away from that recognizing, that 4000kg is a load which can't be transported as long as there is some rest of economical thinking. People doing that didn't like the idea making wrong promises.
Both ex-Omega aeroplanes were former pax aircraft modified for cargo transport by an American STC holder. There is nothing special in that. Finally, it is just some kind of change in the cabin lay out developed, when Embraer couldn't offer a cargo version.
Guess when the authority checks your EMB-120 flight plan taking off with 4000 kg, you might have some trouble because the distance you can fly with that is almost walkable.... ;-)