Pure curiosity:
So if you want to mix material from different expansion packs in Tyros 4, what do you do?
You can't, it's as simple as that!
Even if you only want to use just one custom voice from a Yamaha Tyros4 pack, you must load the whole pack!
That is one of the big advantages of YEM since you can make effective use of the expansion memory by only loading the voices that you want.
You can have more than one pack loaded on the Tyros4 unlike the single PPI from YEM based instruments, and of course third party custom voices will be in the form of individual UVN/UVDs which can be loaded singly and freely mixed. Packs and individual UVN/UVDs can also be freely deleted, although if the item being deleted is not the most recently loaded one, it is necessary to 'defrag' the module to actually recover and reuse the memory, which can take some time. In fact it can actually be quicker to just reformat the memory thus deleting all the existing content, then loading everything again!
However there are system limits on certain parameters which can prevent you from loading additional packs, even if there is enough empty memory on the expansion board. The most problematic of these parameters is the so called 'Wave Count' which was increased in the final firmware update for Tyros4 from 8192 (mono) to 9999, but even then I am left with plenty of space (239MB) on my 1GB expansion board for another large Yamaha pack, but only 1050 'Wave Counts', and since the pack requires more than this, the OS
won't load any of it!
It appears to me that YEM is a much better system in terms of efficient use of expansion memory, but I can't see Yamaha ever making it available for Tyros4 (or earlier), even if it were possible to do so!
Regards
Ian