4 way fight or 2 way fight?
"so the field seems clear for a four-way battle between Apple, Google, Microsoft and RIM"
Windows Phone 7 and 7.5 are an utter joke, and the only reason it's gets listed is it's Microsoft, the reality is that it's a stillborn platform (like Bada) , and I would argue that RIM is also dying too..
It's a straight two-way fight Android / iOS battle, that Android seems to be dominating. (hence why Apple are resorting to litigation as their only remaining defence).