Lights 3 and 4 flashing is an NTC (temperature sensor) fault.
What you need to do is remove the back panel an from there you will see the back of the drum.
There you will see the heater element. In the middle of the heater is the NTC. It will have 2 thin wires going to it. Check the wires haven't come out or broken in the plastic block.
If the wiring is fine then you need a multi meter to test the NTC. Check the resistance of the NTC as it should be around 8 Kohms. If faulty then the NTC will need to be replaced.
But if the NTC and wiring are fine then im afraid you are looking at a faulty control board at the back of the washer.
If you need anymore info just get back to me.