Although I have a CPA meningioma, not an acoustic neuroma, I went four years in wait and watch (and even skipped an MRI one year because I was so sure that nothing had changed). Well, the MRI after that showed that the little bugger was starting to grow slightly, but definitely getting bigger. I went for a consultation with a new neurosurgeon who recommended radiation treatment while it was still safe to do so. She said I could wait but that if the tumor became much bigger, the radiation option would no longer be available to me. In answer to your question, being on watch and watch for four years does not mean that your tumor will not start growing at any time.