Channel Four's in guide shown by TV, but not Five (7)

I believe the answer is:

rivulet

I'm a little stuck... Click here to teach me more about this clue!

'channel four's' is the definition.
'rivulet' can be an answer for 'channel' (thesaurus). I am not sure about the 'four's' bit.

'in guide shown by tv but not five' is the wordplay.
'in' is an insertion indicator.
'guide' becomes 'rule'.
'shown by' becomes 'iv' (I can't explain this - if you can you should believe this answer much more).
'but not' suggests deleting specific letters.
'five' becomes 'v' (roman numeral).
'tv' with 'v' taken out is 't'.
'rule' placed around 'iv' is 'rivule'.
'rivule'+'t'='RIVULET'

Can you help me to learn more?

(Other definitions for rivulet that I've seen before include "burn" , "fast stream" , "Brook" , "Big stream" , "Vile rut for small stream" .)

I've seen this clue in The Telegraph.

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