P3D v4 works with files containing the extended character sets, and Ideal Flight Pro reads and writes these files correctly as they should be written without changing the format of the files.
However, for the time being, Ideal Flight will not work for a "User Name" containing non-ANSII characters. Windows User names like FS, Joe, or Steve are OK. Windows User Names containing characters such as "документ" are not supported.
This is a decision taken, based purely on support capability - it's unlikely I can offer proper support for those systems configured for extended User names.
In short, for the meantime, Ideal Flight only works with Windows User names containing the base character set.
In the future IFPro will be enabled to install onto those extended User names, when demand increases.