BBC Four

Background
BBC Knowledge was an early BBC digital channel available in cable, satellite and terrestrial broadcasting prividing a programme of documentary, cultural and educational television. The channel would close down on March 2, 2002 and was replaced by BBC Four (although it was planned to launch in 2001 and had plans dating back to 2000). BBC Four's programming consists of a similar focus to that of BBC Knowledge, and runs from 7PM to 4AM on the CBeebies channel. BBC Four, along with CBBC and BBC Radio 4 Extra are likely about to end their terrestrial networks by 2025.

1st Logo (June 1, 1999-2000)
Logo: We see cartoon characters climbing ladders on an background, while showing items like a magnifying glass, a telescope or a roll of paper. Eventually, the screen is centered in one ladder, while the BBC Knowledge logo appears below.

FX/SFX: Hand-drawn, cartoonish animation.

Music/Sounds: TBA.

Availability: Extinct.

2nd Logo (1999-2001)
Logo: TBA.

FX/SFX: TBA.

Music/Sounds: TBA.

Availability: Extinct.

3rd Logo (2000-2002)
Logo: We see several sorts of circles or rings zooming quickly towards the screen while the background changes color: First, a ring made of flame on a background, then a ring of  numbers on a  background, a water puddle on a  background, a spinning ring of metal on an  background, and finally a  eye iris on a background of the same color. All of this repeats.

FX/SFX: The quick zooming, the background changing colors

Music/Sounds: An "orchestral" version of the previous logo's music.

Availability: Extinct.

1st Logo (March 2, 2002-September 10, 2005)
Logo: On a black background, there are CGI shapes, usually having or  lights, that react to the music and the voice of the announcer. The BBC Four logo (a black box with the BBC logo at the top and "FOUR" in Gill Sans at the bottom) is seen in the bottom right corner.

FX/SFX: The shapes reacting to the music.

Music/Sounds: Depends on the variant, though a continuity announcer is present.

Availability: Extinct.

Legacy: As this ident's animation was synced to the audio of the announcer and/or music, it stands out with the fact that no two idents were ever exactly the same.

2nd Logo (September 10, 2005-)
2005-2021=

Logo: 4 different sections of the screen are shown, with the illusion that these are part of the same video. In the middle, the BBC Four logo can be seen. Several different idents have been made, one of the most famous being a "single" video of a lake with mountains in the distance, and ripples appear both in the lake's reflection and in the sky.

Later Variant: Beginning on October 20, 2021, with the introduction of the BBC's new logo, the idents were altered a bit. The square in the middle is gone, with the new BBC logo on top and the text "FOUR" in the BBC Reith font on the bottom, like the new BBC logo. The "Subtitles" graphic is now placed in a full widescreen picture, rather than being in its original place of a 4:3 picture.

FX/SFX: Live action.

Music/Sounds: Depends on the variant, but there's a continuity announcer present. The main theme uses loud synth squeaks, and ends with a guitar note.

Availability: Current, being used for over 17 years as of October 2021, making it one of the longest-lived idents seen on British TV.