#! perl -sw
use strict;

my $file = $ARGV[0];

open I, '<', $file or die $!;

my @outs;
open $outs[ $_ ], '>', "$file.$_" or die $! for 1 .. 4;

my $out = 1;

while( <I> ) {
    print { $outs[ $out ] } $_;
    if( /^5/ ) {
         ++$out;
         $out = 1 if $out > 4;
    }
}