please forgive my stupidity, I totally forgot about AS grades carrying through!
Are you SURE that your university will accept all 4 grades? As a standard rule they make 3 grade offers. I'd advise you to check this directly with your uni before proceeding unless it explicitly states so on the prospectus or website.
As I said, contextualised data is used so it depends on other people's results. Is the subject you wish to study one of those predicted Bs? I'd recommend pushing for an A in that as this will strengthen your application further and put you above other applicants who may or may not miss out by the 10 UCAS points you fear you will. The weighting of the UCAS points makes a difference in their decision. If they have someone like yourself who got an A in their chosen subject against someone who got a B, given no other differences they would most likely accept you over your opponent. The same applies for a B vs a C etc etc.
As I said, I can only speak from my own experience so for example I study French and German, and want to go and read French and German at uni, so these grades are obviously more important than the 3rd, irrelevant subject, although they still look for all round intelligence.
I would also recommend bolstering the EC side of your application as this makes a difference for unis (from my own experience.) For me this was gaining work experience in French- and German-speaking environments, as well as in France and Germany, spending time in the country, helping out with language club, etc etc. Also try and chum up to your teachers as their references are important in a UCAS application!
Hope this helps