To be honest, it sounds as though you know what you're doing already (although I personally wouldn't enter any son of mine for Sutton Coldfield - or to give it it's full name, Sutton Coldfield Grammar School for Girls
As you suspect, 4a in SATs is a good starting point. There are no guarantees though - there are many, many more children with 4a at the end of Y4 than there are GS places in/around B'ham, but equally there are many children with far less encouraging predictions that get GS places; they're measuring different things, so it's a good place to be but I'd advise that you make sure he really understands that there is no certainty of a place.
If you're considering QM and BV you're presumably also within reach of KE Aston, and you may also be within reach of KE Camp Hill and KE Five Ways if you're close to a Green Bus route (http://www.thegreenbus.co.uk
Be warned that probably most tutors (well, we're on our second and definitely goes for both of them) appear oblivious to the style and content of the CEM tests that are used for both QM and BV/KE and will feed your son copious amounts of NFER-style Verbal Reasoning which is not very relevant (the maths and NVR they will provide will probably not be all that well matched either, come to that, but there's not much that's more appropriate as far as I can tell). However, I suspect that there's a lot of value simply in the "learning to think" that results (hopefully! it did for our son but I can't see much sign of it in our daughter
) so I wouldn't necessarily write off the idea of using a tutor, but I'd recommend you to read fm's posts on suitable test prep material and decide whether you want to either top up what the tutor does, DIY instead (assuming you can work with your child without World War 3 breaking out - I admire those who can), or accept what the tutor provides (perhaps with a little helpful advice!).