Please try this one:
//if your payment date is 15-4-2022 then you have to add 4 days in carbon objects.
$dueDate = Carbon::now()->addDays(4);
Payment::select( 'next_due_date')->whereDate('next_due_date', $dueDate)->get();
//or
Payment::select( 'next_due_date')->whereDate('next_due_date', date('Y-m-d', strtotime("4 days")))->get();