In a speech, Director-General Tim Davie said that in order for the BBC to stay relevant "we need to evolve faster and embrace the huge shifts in the market around us."
He said: "When I took this job I said that we needed to fight for something important: public service content and services, freely available universally, for the good of all. This fight is intensifying, the stakes are high."
CBBC first launched in 1985 as part of the BBC programming block, before becoming its own channel in 2002. BBC Four also launched in 2002, while Radio 4 Extra started as BBC 7 in the same year, before relaunching in 2007 under the new banner.
CBBC has been home to hit children's shows including The Sarah Jane Adventures and Tracy Beaker, while BBC Four has produced Detectorists and Twenty Twelve.
Other changes announced as part of the plans include the creation of a single, 24-hour TV news channel serving UK and international audiences, shifting a number of World Service languages to be digital only, an ambition to reach 75 per cent of BBC viewers through iPlayer each week and new on-demand content and formats for news and current affairs.
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This will involve a £300m reinvestment to drive a digital-first approach, through changes to content and output and additional commercial income. This includes shifting significant amounts of money into new programmes for iPlayer and making investing money in video and digital news rather than broadcast news.
The news comes at a time when public service broadcasters are under continued scrutiny, with Channel 4 due to be privatised under new Government plans.
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