Fourth television network

The October 1986 launch of the Fox Broadcasting Company was met with ridicule. Despite the industry skepticism and initial network instability (due to its early struggles in launching hit programs), the Fox network eventually proved profitable by the early 1990s, becoming the first successful fourth network and eventually surpassing the Big Three networks in the demographics and overall viewership ratings by the early 2000s.

Question: by the mid-1980s fox had emerged as a viable fourth network
Answer: no