I like your categorizations…a few suggested additions / refinements…

From Four to Two & A Half (1948-1959): 1948 was the first season that all four original networks were up and running across seven nights with programming. By 1955, DuMont was gone and ABC was widely referred to as “half a network”, largely due to its perennial also-ran status in the Nielsens and its weaker affiliate station mix.

The Man with the Golden Gut (1976-1979): As much as Happy Days was archtypical of the new ABC, this era of ABC's sudden dominance coincided perfectly with the rise (and fall) of Mr. Fred Silverman as programming chief…”the man with the golden gut” per Time Magazine. Those Tuesday comedies rolled up massive nightly wins for the network…ABC went from winning 2 of 33 Tuesday nights (CBS won 26) in the 1974-75 season, to winning 13 of 22 in the 1975-76 season to steamrollering the other nets winning 27 of 30 nights in the 1976-77 regular season. But were only part of the story of ABC dominance of this era. The other half of the Silverman era was his complete embrace of Mr. Aaron Spelling as an exclusive producer to ABC. Mr. Spelling gave Mr. Silverman solid, dependable hits that he could deftly slot across every night of the week beyond Tuesdays.

Households Cable Up & Mini-Networks Erode the Pie (1991-2004): Beyond the unceasing proliferation of cable networks, both ad-supported and pay-tv, these 15 years also saw mini-networks sit down at the table and start to eat the pie that had made the Big Three very fat and complacent. FOX was the first and is now the strongest mini-network (its limited primetime and daypart programming still keeps it from being a full network in my books). But the broadcast pie also began to be eaten by the WB, UPN, PAX and their progeny CW, MyNetworkTV and i/ION. The Spanish-language networks (Univision, TeleFutura, Telemundo, Azteca) also arrived, expanded and took away viewers, especially in major markets. And all through this era, PBS remained a determined broadcast competitor to the networks and hasn't lost market share to the same extent as the Big Three because of its loyal, public television audience.