One way:

>perl -wMstrict -le
"my $s = 'xx123xx4321xx56789xx987654xx';
 ;;
 my @n = $s =~ m{ (?<! \d) \d{4,5} (?! \d) }xmsg;
 printf qq{'$_' } for @n;
"
'4321' '56789'
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