That's not implausible. Industry watchers have certainly not ruled out one more round of mergers, and it would probably end up being balanced in the manner your describe.

At some point the trust-busters have to decide how many little airlines is too few.

On the other hand the little airlines may not be able to thrive or even survive competing against the economy of scale of the big four. Maybe the trust busters will bow to the inevitable and not force the shareholders to ride doomed small companies into the ground when saviors exist who would buy them. Especially since the sudden liquidation of even a smallish major airline would be disruptive to travel and the economy, especially at the wrong time of year. Also considering that all training bandwidth available to the big three might be consumed with replacing retirees for a number of years... if you want to keep the pilots and planes of a small major flying, may be best to ensure that operating certificate continues to operate, vice liquidation and a fire sale of assets.