Using hostname output in linux with sed

I am trying to use part of the hostname output in Linux and use it in a file using sed. For example, I am using this command as hostname | tail -c 4 which shows me output as last 4 numbers and then use this output and substitute it with another text inside a file. Assuming part of the hostname command shows with tail -c 4 option, "1234" How can I take it this further and use it to replace it with another file? I can do this manually by: hostname | tail -c 4 ; sed -i 's/oldtext/1234/g' filename.txt but not sure how can I achieve this with script? Any ideas?