What are you specifically looking for ? A given SDK documentation (SDK 4.2 in your case) is applicable for a specific release of BIG-IP. Therefore, all interfaces/methods described in the SDK documentation should be available for that release.
I don't quite understand what you mean by "to try and fail on a 4.2 box" ? Please advise.
Specifically, Monitor2 is a redesigned interface that allows you to more efficiently work with monitor templates, associations, and instances. Functionally, Monitor2 exposes the methods the same way that you would work with monitors via Bigpipe or the GUI, so that all 3 interfaces will be more consistent.
You're in the right place. In the navigation applet on the left, click on Release Documentation->Release Notes. It should list out what interfaces are new or have been enhanced. I just looked, and it doesn't list out the new methods like I've said though.