UNIX for Dummies Questions & Answers

sed '
1{h;d;} # line 1, copy to hold space
2,3{H;d;} # lines 2..3, append to hold space, separated by \n
# all remaining lines
H # append to hold space
g # copy to pattern space
s/[^\n]*\n// # delete first line
x # swap pattern space <-> hold space
s/\n.*// # delete all but the first line
# last line, insert the text
$i\
NEW LINE
$G # last line, append hold space to pattern space, separated by \n
' file.txt

$ sed -e ':l;N;$!bl;s/\(\n[^\n]*\)\{4\}$/\nNEW LINE&/' file.txt