I was the same. I moved to grammar and wasn't allowed to sit a 4th AS as I only had 7 GCSEs.
Your options are not limited very much at all without a 4th AS, only Bristol, Oxbridge and unis of that ilk are out of reach. As an example, I'm applying to King's, St Andrews and Nottingham this year, all excellent unis and base their offers on having 3 subjects to A2. In fact, KCL has already confirmed to me that their offers are only based on your A2 predictions and the additional AS carries no weight.