The increased competition will force ABC, CBS, Fox and NBC to settle for their lowest average rate hikes in three years during the "upfront" selling season, Wall Street analysts say.
During the upfronts, networks preview shows for their fall schedules, trying to persuade advertisers to buy billions of dollars worth of commercial time in advance.
The broadcasters still command premium ad prices because they reach an audience that is far bigger than the viewership of any single cable channel. Upfront rates likely will rise by 6 percent on average, as the broadcasters book about $9 billion worth of ad inventory during the upfronts, Barclays Capital estimates.
While 6 percent is well ahead of the 1.3 percent annual inflation rate, it is lower than the rich gains networks enjoyed in recent years. Upfront ad rates increased by 7.5 percent last year, and by 11.1 percent the previous year.
"For the networks, they probably feel very challenged that they have more competitors and are facing lower ratings," said Mark Fratrik, chief economist at media research firm BIA/Kelsey.
"Advertisers have many more places to go to, so broadcasters are probably a little reticent of trying to push stronger (rates), even with this stronger economy," Fratrik added.
Viewers' biggest distraction is cable TV, which is churning out more hits that lure eyeballs from the Big Four. AMC's zombie thriller "The Walking Dead" and the A&E reality show "Duck Dynasty" haul in broadcast-sized audiences. "Walking Dead" averaged 10.7 million viewers this season, more than all but the top 12 shows on broadcast TV.
Online video players such as Hulu and Google Inc's YouTube are jockeying for ad dollars, and viewing hours are growing on Netflix, the streaming service that is making a big push into original programming with shows like political thriller "House of Cards."
Plus, networks don't yet get full credit in Nielsen ratings for the viewers who catch their favorite shows online.
So far this season, combined prime-time ratings on the four broadcasters declined 7.5 percent, the biggest year-over-year decline in six years, according to Nielsen data provided by Horizon Media and based on live viewing and those who record and watch the show the same day.