Although the program architecture of Prepar3Dv4 has moved from 32-bit to 64-bit, one should not henceforth assume that its associated environment (sky, cloud, sea, ground) texture data would have been upgraded accordingly.
Actually they are largely the same as those used in previous P3D versions, and in FSX as well. Only a limited number of files I found so far have been changed or converted into a different format.
This is good because it means that it is possible for me to restore my previously saved enhanced environment texture backup (put together from different addons) onto P3Dv4 now, without waiting until those addon programs becoming version 4 compatible later.
Some restores are simple copies and pastes. Some are done via the changing of the installation paths in the addon programs. For example, I’ve designated the folder location of P3Dv3 in REX’s Texture Direct by the path of P3Dv4 as shown in the image below. In this way, the enhanced texture files will be restored and installed onto the P3Dv4 folder instead.
One thing should be noted that it is extremely important to create a backup of the original P3Dv4 textures first (say via the program’s backup feature). This will protect us from accidents in case something goes wrong.
True, it is very likely that the addon program once updated will be optimized for 64-bit and may come with fixes for various glitches as well. But as a temporary measure, this is what I did and I found no problem so far.
NOTE that your mileage may vary and the risk is on your own.