FLYGVA

Indeed. On the one hand, this means you could leave the airport at 14:00 and your plane does not land at 14:00.

The easiest option if you use public transportation is to take the
bus "TXL" from the airport to Berlin "Hauptbahnhof" and change there into a train towads "Schönefeld HBF"

E.g.

Bus "TXL" to "S+U Alexanderplatz via Hauptbahnhof"
leaving the airport at 14:12
arriving at "Hauptbahnhof" at 14:32

(the bus TXL leaves app. every 6 - 8 minutes

Change to

RegionalBahn RB 7 to "Wünsdorf-Waldstadt, Bahnhof"
leaving Hauptbahnhof at 14:45 on track 12
arriving "Flughafen Schönefeld" at 15:05

(alternative departures RB14 at 15:10, RE7 at 15:45, RB14 at 16:10, RE7 at 16:45 [RE = RegionalExpress])

For more information http://www.bvg.de/index.php/en/index.html

They have a journey planner on the right hand side. Enter "Tegel" in field From and "Schönefeld" in field "To"and the time. Click on Search. You will most likely now see all entries matching "Tegel" and "Schönefeld". Select "Flughafen Tegel (Airport)" and "S FLughafen Berlin Schönefeld" from the drop down boxes and click on search connection and al available journeys are shown. There is no direct line, you have to change at least once from bus to train / S-Bahn.

It is easy and convenient, but I am used to public transportation. If you prefer a direct service, a limousine as suggested here, might be a better idea. But given that you have the full four hours, you should be fine.

The bus stop is in front of the terminal and clearly marked by signs in the terminal. There is a ticket vending machine and usually someone from BVG (=Berliner Verkehrsbetriebe, the local public transportaion network) is assisting passengers.