I would recommend putting the comprehensive in 3rd place, and a grammar as a write-in. And make sure the 4th school knows you have put them down - that gives you an easier chance of appealing for it if you need to.
Not sure where you're an appeal panel member Capers.
Over here in Dudley things are a bit of a mess. I can think of five out-of-borough selective schools people might want to put on their form for a girl (two less for a boy). Where I live a further four or five foundation and community schools would make sense too.
Dudley LEA recently increased the number of options to six but some children are still having their educational options narrowed , it seems to me, for no good reason.
If I had to apply via an LEA form with only three options, or Dudley had kept that system, I just don't know what I'd have done. Sympathies to those suffering that ridiculous restriction.
You can, technically, appeal to any school whether you've put them on your application list or not. Successful appeals are few and the initial schools waiting list is where you really want to be – and for that you need them on your application form.
Unfortunately writing to a Community school to tell them you'd've liked to have put them on your list couldn't possibly have influence here; the same letter to a foundation school would just be considered an insult – “Why didn't you put us first!”.