You can fetch all the records as a 2D array ref and then directly access record 4. Each field of record 4 can then be accessed using the elements of the 2nd array.

#get array ref
my $results=$sth->fetchall_arrayref;
#rec 4 is array element 3 due to the 0 element
#my $field=$result->[row][fieldno]
my $field1=$results->[3][0];
my $field2=$results->[3][1]; 
my $field3=$results->[3][2];
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Alternatively you fetch an array or array ref row by row with

my @results=$sth->fetchrow_array; 
my $arrayref=$sth->fetchrow_arrayref;
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You can't retrieve record 4 only without a loop unless your sql statement returns only one record. You can usually specify the rows to return with a LIMIT clause