Indent lines of text following given rule

If you just want to indent the first line and then every 4th line after that, you can use awk:

$ awk 'NR % 4 != 1{$0="    "$0};1' file 
OF Mans First Disobedience, and the Fruit
    Of that Forbidden Tree, whose mortal tast
    Brought Death into the World, and all our woe,
    With loss of Eden, till one greater Man
Restore us, and regain the blissful Seat,
    Sing Heav'nly Muse, that on the secret top
    Of Oreb, or of Sinai, didst inspire
    That Shepherd, who first taught the chosen Seed,
In the Beginning how the Heav'ns and Earth
    Rose out of Chaos: Or if Sion Hill
    Delight thee more, and Siloa's Brook that flow'd
    Fast by the Oracle of God; I thence
Invoke thy aid to my adventrous Song,
    That with no middle flight intends to soar
    Above th' Aonian Mount, while it pursues
    Things unattempted yet in Prose or Rhime.
And chiefly Thou O Spirit, that dost prefer
    Before all Temples th' upright heart and pure,
    Instruct me, for Thou know'st; Thou from the first
    Wast present, and with mighty wings outspread.

Or perl:

$ perl -pe 's/^/    / if $. % 4 != 1' file
OF Mans First Disobedience, and the Fruit
    Of that Forbidden Tree, whose mortal tast
    Brought Death into the World, and all our woe,
    With loss of Eden, till one greater Man
Restore us, and regain the blissful Seat,
    Sing Heav'nly Muse, that on the secret top
    Of Oreb, or of Sinai, didst inspire
    That Shepherd, who first taught the chosen Seed,
In the Beginning how the Heav'ns and Earth
    Rose out of Chaos: Or if Sion Hill
    Delight thee more, and Siloa's Brook that flow'd
    Fast by the Oracle of God; I thence
Invoke thy aid to my adventrous Song,
    That with no middle flight intends to soar
    Above th' Aonian Mount, while it pursues
    Things unattempted yet in Prose or Rhime.
And chiefly Thou O Spirit, that dost prefer
    Before all Temples th' upright heart and pure,
    Instruct me, for Thou know'st; Thou from the first
    Wast present, and with mighty wings outspread.

In both cases, we are adding 4 spaces to the beginning of the line if the current line number module 4 is not equal to 1, which means we will do it for all lines except the 1st, 4th and so on.

In awk, NR is the line number, and $0 is the contents of the line, so NR % 4 != 1{$0=" "$0}; means "add 4 spaces to the beginning of the line when the current line number modulo 4 is not equal to 1". The final 1; is just shorthand for "print".

In Perl, $. is the current line number, and s/old/new/ is the subsitution operator which will replace the first occurrence of old with new. So s/^/ / if $. % 4 != 1 means "replace the beginning of the line (^) with four spaces if the current line number modulo 4 is not equal to 1". The -p means "print each line of the input file after applying the script provided by -e".

Here is the exact same perl command in a more verbose and easier to understand version:

perl -e '
open(my $fileHandle, "<", $ARGV[0]);
my $lineCount=0;

while(my $line = <$fileHandle>){
   $lineCount += 1;
   if ( $lineCount % 4 != 1 ){
       ## or $line = "    " . $line
       $line =~ s/^/    /
   }
   print "$line";
}' file

Or, almost identical:

perl -e '
open(my $fileHandle, "<", $ARGV[0]);
my $lineCount=0;

while(my $line = <$fileHandle>){
   $lineCount += 1;
   unless ( $lineCount % 4 == 1 ){
       $line = "    " . $line
   }
   print "$line";
}' file