You need to take her to the vet immediately to get thoroughly checked out, including hearing and sight. Highly possible there's a physical cause.
Other than that, how old is she and how long have you had her? What's her background and how much exercise does she normally get? Have you ever done any Cesar Milan type training on her such as alpha rolls to establish yourself as the alpha dog? PLEASE NOTE that is a question, NOT a suggestion.
I wonder if her ears are sore. I have a spaniel, and when he had an ear infection he was very 'mouthy' if we approached his head. He was a much younger pup than your girl, though, so we just thought he was teething at first.
We've got a cocker spaniel x and she was exactly the same, we got her at 8 weeks, but we think she was taken away from mum to soon, we gave her lots of praise when she did ANYTHING good, we also got her a cuddly toy which she loved and started suckling it, she soon got over the aggression, and now at the age of 3 she is an amazing dog. Hope that helps.
4 weeks is far too young to leave mum. Note you didn't answer all my questions. And yes, I most definitely am not advocating CM, his methods can cause an aggression (through fear) problem where none existed.
not sure what cesar milan is TBH, the training which we do with her is based on ignoring bad behaviour and as rewarding the good with treats and praise etc. At the moment the treats are her complete dry food, as I'd prefer to get some of that into her rather than bits of sausage IYSWIM. She gets a short walk in the morning and a longer one off the lead and a good run at night. At the weekends she has two of these runs. I've got a bag of the Burns food now so will start to swap her over. Didn't get to the vet - RL got in the way. So will make this a priority for tomorrow.
It's hard to decribe her behaviour but the majority of the time she is happy, wagging tail, happy to play etc but without any warning she can just turn, her eyes are wide, teeth showing, heckles up, growling etc.