use strict;
use warnings;

my @one_lines;
open my $one,"<","file1.txt" or die $!;

while (<$one>){
   chomp;
   my (@fields)=split /:/;
   push @one_lines,{KEY=>$fields[0], F4=>$fields[3]};
}
close $one;
#------
open my $two,"<","file2.txt" or die $!;

while (defined(my $line=<$two>)){
   chomp $line;
   my @fields = split /,/,$line;
   my $matched_line={F4=>""};
   for (@one_lines){
     next unless $line=~m/$_->{KEY}/;
     $matched_line=$_;
     last;
   }
   $fields[5] = $matched_line->{F4} if $matched_line->{F4};
   print join(",",@fields),"\n";
}
close $two;