version 4 is not 'out'. It is a first alpha. it will take 6-8 months before a GA release will be there.
Regarding the api and jpdl, the API will not be backwards compatible, but from 4 on it will be. Otoh, if you encapsulate things in your own 'process access object' like a dao, your changes will be localized.
jPDL will change as as terminology from BPMN will be used where appropriate. A conversion will be provided though that will do 80-100% automatically, depending on what you use. Details are not known yet