2yr old temp up to 39.4 up,and down for 4 days,gp said teething.vomiting couple of times,day4 bit of diarrhoea,very lethargic,drinking plenty,no ras?

Not teething: A temperature that high is not due to teething. It it's more likely due to a viral illness. Your child sounds fairly sick. With those symptoms for four days, you need to take him to see a doctor asap.

Viral: Those symptoms sound more likely to be a viral illness. Teething may cause low grade temps but not that high. If you bring the fever down with ibuprofen, and he feels better, you can probably watch him for a couple of days. If you bring the fever down and he still feels terrible, I'd get more worried and have him checked again.

Fever in 2yo: This is not due to teething. It does not produce nausea, vomiting and diarrhea and a fever of 39.4 is much too high to try and attribute it to teething. You need a second opinion from a pediatrician.