New to cooking - how long to cook chicken breast pieces in a wok?
Unfortunately, temperature is the only thing you can go by: the phrases 'medium heat' and '4/6 on the hob' don't really give us any information, and there could be significant variations in cooking time due to the shape of the wok, the starting internal temperature of the chicken, the shape of the breasts, and so on. 

With chicken, the entirety of the meat (down to the center) has to be exposed to a high enough temperature for long enough that any bacteria that may be present will be killed. This happens essentially instantly at 165 F, though one can cook it at lower temperatures for longer periods of time for the same effect (this is much harder, because you need to sustain a relatively high temperature, in the range of 145-150, throughout the entirety of the chicken for something on the order of half an hour). Whether or not the meat appears bloody is irrelevant, so long as the meat has been cooked to the right temperature.

There are plenty of general guides on determining when chicken is done: for instance, Pan frying chicken breasts? and How to know when chicken breast has cooked through?. You will likely need to experiment or get a meat thermometer.