I've made this available elsewhere:
"Under the hood it’s a substantial difference in terms of handling the new P3D platform. Making one Ideal Flight app to run on P3D v2/3 and also P3D v4 was impractical and presented no option other than to create the new version for the 64bit platform and make considerable changes for the going ahead.
In use all Ideal Flight versions maintain the same operational feature set as near as is possible and applicable between simulators. The application remains familiar and most user records and profile data can be copied between versions as well.
Most recently Ideal Flight routes capabilities have been improved slightly and the data handling extended to make routes from fsAerodata as well as several different AIRAC navigation database types, not forgetting the sim native data. Ideal Flight compares data sets to produce the reliable data subset most useful to the chosen user aircraft within the current simulator scenery setup.
Ideal Flight continues to provide the most reliable routes and plans for use of the FMC and GPS systems in the FSX/P3D add-on market today."
I'm very grateful to everyone helping with Ideal Flight - thank you!