Shamefully, I am not currently providing any specific API version on my API requests. When will the default become v5? And is current default v3 (I had been assuming that, but given that another post implies that there is a v4 behind the curtain somewhere, I was not sure if current default might be v3 or v4).
p.s., I promise to mend my ways and use version number going forward … 
v3 is the default and will be until it’s deprecated, v1 and v2 removed, and v5 becomes the default some time next year.
To add onto what @3ventic has said, on February 14, 2017, v1 and v2 will be removed and v3 will be moved to a deprecated state, however it will remain the default until it is removed on February 14, 2018 - at that point, v5 will be default.
This info was pulled from the blog post and this post from Dallas.
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This is all covered in the discussion and FAQ thread. Closing this one. If you have further questions, please post there for the benefit of everyone else that might have a similar question. 