I recently installed a Miele T1 and used this to cap my dryer vent. IMPERIAL 4-in Dia Galvanized Steel Round End Cap Item #607429 Model #GVL0270 from Lowes

Also, if you are capping the interior side, consider some insulation at either end of the ducting immediately behind the cap or caps. Helps mitigate against wind and outside temperatures, particularly if you live somewhere that gets very cold in winter or very hot in summer or both. Use something like fiberglass batting rather than foam sprays so it can be easily removed should you or some later resident want to go back to a vented dryer.