Hi Simon what Jez said is right, the way to fill the oil in the bike is to, drain the Oil tank, Drain the engine, take the filter out and replace it.
Then put in about 2 litres of oil into the oil tank, start the engine and run if for a short while until the oil level in the oil tank drops and then add the rest.
You are lucky that you have not caused hydraulic lock in the motor putting in 4 litres of oil.
The Oil pressure switches are also prone to failure. So make sure it works when you get the oil right.
There are plenty of car type oil pressure switches that fit the bike, all you need is the right thread and pressure settings.
The manual should tell you what pressure the oil switch operates at. You need a normally closed switch. This means that the oil pressure switch contacts are closed when the pressure is below the setting point. So the light will come on when the engine is off or when the pressure is below the range when the engine is running.
Cheers
Pete