The airport pack and airports from ORBX can work in v.4, BUT. most of the animated objects will not work. The Objectflow.dll, which governs animations, needs to be updated. When it is, ORBX will release all the airports.
Some airports can be installed in v4, but they will not have objectflow.dll ORBX has not released any airports because of this (Bar Harbor is listed above, but has not been released). If it works, it works, but it is luck rather than design.