I have NOTION 3 and NOTION 4 running on the Mac Pro here in the sound isolation studio, and I can open NOTION 3 scores in NOTION 4 and open NOTION 4 scores in NOTION 3, but when NOTION 4 is running in 64-bit mode I use 64-bit VSTi virtual instruments and they are not found or recognized by NOTION 3, which is 32-bit only, so while in some respect you can switch from NOTION 3 to NOTION 4 and vice-versa in a general way, when NOTION 4 is running in 64-bit mode and you use VSTi virtual instruments and VST effects plug-ins, these will not be available if you open the score in NOTION 3 . . .
The strategy I use is based on which VSTi virtual instruments are in score, and if the VSTi virtual instruments are 32-bit, then I use NOTION 3, because it works nicely and I know all the applicable rules and caveats. If I use 64-bit VSTi virtual instruments, then I do the work in 64-bit NOTION 4. And the only time I run NOTION 4 in 32-bit mode is to help someone with a problem which is specific to the 32-bit version of NOTION 4, and once the help is provided I switch NOTION 4 back to 64-bit mode . . .
Summarizing, all my 32-bit scores were done with NOTION 3, and I continue to use NOTION 3 to work with them, but for new scores I use 64-bit NOTION 4 and 64-bit VSTi virtual instruments and VST effects plug-ins, since it nearly always is easier to go forward, which also is the case with the DAW application, where if a project was started in 32-bit mode I use Digital Performer 7 (MOTU) if I need to ReWire or whatever but otherwise I convert the 32-bit projects to 64-bit and use Digital Performer 8 . . .
Being able to go back-and-forth is handy at times, but the primary strategy is to go forward; to move ahead; and then to whip it good, which is fabulous . . .
Fabulous! 