I found a dead simple solution to this.
In your main IL2FB/PF/46 directory, wherever it is, and whatever it's called, create two zip files. I called them "reassert 408.zip" and "reassert 409.zip"
Into these zips place the following files from each patch:
files.SFS
il2_core.dll
il2_corep4.dll
mg_snd.dll
mg_snd_sse.dll
When you want to play under 4.08, simply 'unzip to here'.
When you want to play under 4.09, simply 'unzip to here'.
A quick and painless switch, endlessly repeatable, as far as I can see.
There are probably complications that I am unaware of, and difficulties for others, but it works for me every time.
Regards.