regex working on Vim but not in sed

Hello,

I have this substitution regex working properly when executed from
inside Vim.

%s/\(instance_hostname.*\n.*\n.*\n.*\)myhostname.net/\1newhostname.net/
g

Since it has to be executed on hundreds of files I've tried to convert
it into a sed one liner but with no success.

I've tried this combinations (changed quoting, [un]escaping
characters):

sed -i 's/\(instance_hostname.*\n.*\n.*\n.*\)myhostname.net/
\1newhostname.net/g' myfile.txt
sed -i "s/\(instance_hostname.*\n.*\n.*\n.*\)myhostname.net/
\1newhostname.net/g" myfile.txt
sed -i 's/(instance_hostname.*\n.*\n.*\n.*)myhostname.net/\
\1newhostname.net/g' myfile.txt

I'm obviously missing something, but don't know what.
Can somebody help me please?

Thanks in advance.
.

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