open my $file, "<" , "Filename" or die "$!";
my @buf;

while (<$file>){
   chomp;
   s/^\s*//; # Drop leading blanks
   s/\s*$//; # Drop trailing blanks
   if (0 == $. % 4){
     # Processes the contents of @buf
     # it appears that something like this is what you need:
     print join(" ",@buf, $_), "\n";
     @buf=(); # Reset it
   }else{
     push @buf, $_;
   }
}
close $file;
@buf and print @buf; # In case there is left-over stuff.