@Richard238 can you check the SD card isn’t physically damaged at all? And it’s definitely correctly seated in the card slot?
And I don’t suppose you have another Raspberry Pi you could try the SD card with?
It sounds like it could be an issue with a corrupted file on the SD card or another issue with the card itself or the connection with the SD card in the slot, based on the lights flashing.
This is also in the boot problems thread:
Note that 4 flashes could be an indication of a more or less broken SD-card connector. If Databit 1 is connected, but one of the other three Databits doesn’t make contact, so the SD-card only works in 1-bit (SPI) mode, then this will lead to the four flashes error! Check if all pins of your card holder make good contact with the card!