I took a 4th generation test at 3 weeks which came back negative. However today at 4 and half weeks I have started to get a bit of a sore throat and had diarrhoea the last few days.
Do I need to get tested again? How conclusive is the test?
A 4 gen is conclusive after 4 weeks, so you should test again. Symptoms can be from various ailments thus they are not diagnostics so no one here pays attention to them.
No, and it is too important to go with % guesses, so I don't want to create false hopes or fears. If I was you I would avoid googling because there is no diagnostic there, so that can only increase anxiety.