There are 5 of us in my squadron. One or two have open-ended orders with no end-date. At least one has the optional 4th year mentioned. Mine says three years with no mention of a fourth; I too have wondered if I could get it. When I asked the recall office (May time-frame), they weren't sure.

I would guess that like anything military, they can change their mind, and retroactively rescind your orders. Or, they could decide to offer it. I guess we'll see in January.