Problem with sed to insert a new line

[Olivier Regnier]

Hi everyone,

I have a file called "test" with the following lines:
###
a
b

d
e
f
###

With sed, i want to insert the "c" letter after "b" letter. Logically 
simple, but not for me.

Here is my command : # sed -i.old -e "4i\c"

I have this message : sed: 1: "4i\c": extra characters after \ at the 
end of i command

I need help please.

Thank you :)

Olivier.
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