As for the top 4, that still holds true today that they can not merge in any, shape, or form. The DOJ & FCC would rather see one of the top 4 go out of business than to become a super network. I forgot when Fox got included, but it had to be sometime in the mid to late 1990's, since Fox was still a small network in the first 6-7 years of operations.
With Comcast eventually getting full control over NBC, I would not be surprised if they do eventually go pay TV only. They'd rather focus on their cable subscribers & content to their cable networks than TV stations. They could do the same thing with Telemundo as well. If that were to happen, I wouldn't be surprised if their O&O stations go up for sale. For now, I believe there was a condition in Comcast owning NBC, that they still have to maintain the NBC network be available over the air. That can change if the FCC gets their way at removing channels 31 - 36 & 38 - 51 from OTA TV, & reallocate the frequencies for those channels to Verizon & AT&T (the same companies that bought the bulk of the frequencies that were used for channels 52 - 54 & 56 - 69 (channel 55 was originally reallocated for Qualcomm's MediaFLO service, which is now discontinued, & not sure what happened with that 6mhz of spectrum). Even Fox is showing interest in going cable only. They're already showing little interest right now in serving markets under #100 (even if they're letting affiliates run those markets right now).