There is no PMBOK 2013 - there is only the PMBOK version 5 which is currently under review (just a draft) and is expected to be released sometime in 2013.
While there were many changes between PMBOK 3 and PMBOK 4 including the introduction of the concept of Agile and the death of the PM triangle, studying PMBOK 3 greatly helped those who later prepared for the PMP based on PMBOK 4. I expect the same when studying PMBOK 4 to prepare for the PMP under PMBOK 5. By the way, when the new PMBOK is released, the PMP test will not be immediately based on the new PMBOK.
I think that you won't lose anything if you start studying and preparing for the PMP using the PMBOK 4 now.