pseudocode

open (original_file);
$counter = 0;
while(original_file){
  $counter++;
}
my $peace = $counter/4;  # let say the file has an equale number of lines
my $count_for_peace = 0;
open (file_part);
while(original_file){
  if ($count_for_peace == $peace){
    close file_part_handled;
    open(file_part_new);
    $count_for_peace = 0;
  }
  print into file_part_handled 
  $count_for_peace++;
}

my @ch;
for(1..4){
  my $pid = fork();
  if ($pid){
    push(@ch,$pid);
  }
  elsif($pid ==0){
    #read from file 1 and do some processing
    exit;
  }
  else{
    die error;
  }
}
foreach (@ch){
waitpid($_,0);
}