Process Engine JRE shipped with the product is IBM JDK.
You can change the config to point to other JRE but not supported and tested.
Also note that if you plan to use web service client using java code as component step in workflow do remember that there is difference in the web service client implementation in 3.5.x and 4.5.x
3.5.x uses systinet and 4.5 uses Axis.
One of the web service java API method implementation is deprecated in 3.5.x systinet.
During migration do remember to consider this if you are using web service client code in CI.