my $string = "TE=ASR,EV=R,PPF=G1,TG=G1,TMT=P1,RTV=YE,AAT=0,AT=105,ON=Y,CF1=FGOC";

my @fields    = split /,/, $string;
my $per_group = int(@fields / 4);
my $extras    = @fields - $per_group*4;

while (@fields) {
   my $n = $per_group;
   if ($extras) { $extras--; $n++; }
   print(join(',', splice(@fields, 0, $n)), "\n");
}

##</code><code>##

opt1=verylong
opt2=longaswell
opt3=1,opt4=1
opt5=1,opt6=1

##</code><code>##

opt1=verylong,opt2=longaswell
opt3=1,opt4=1
opt5=1
opt6=1