It's 4.75L for the 2.0 model, and something like 4.5L for a 1.2.
Also make sure you give the oil a good 10 minutes to reach its correct level, and make sure the engine is cold when your doing an accurate reading.
Not sure what could be wrong though mate, if you checked its not leaking underneath (pretty sure you'd notice 4litres of oil) then its in the engine, therefore the problem lies with the sensor. Unless of course you just put 4 litres of oil into the washer jet refill
Run the car for a couple of minutes after you testing it, and let it idle. Put your bonnet up and make sure if doesn't sound tappety (knocking). Then try gentley revving it (2k revs) and check for unusual noise. If it idles ok then its good to drive, but check it in the morning to be sure.
You could also try disconnecting the battery to reset the ECU, leave it unwired for 10 minutes and wire it back up, turn your ignition and you'll hear the ECU doing checks before you drive off. Make SURE you know your radio code though if your disconnecting battery - its printed inside the service manual (4 digit code).