Hi Cor,
unfortunately, Tyros4 is NOT compatible to Expansion Manager (YEM). That also means Tyros4 is NOT compatible to Voices/Packs in .ppf format. At this time, there is NO way to convert .ppf packs to the Tyros4 Custom Voice format (.uvn/.uvd), regrettably.
On Peter Schips' website "cms-sounddesign.de" you can find voices for YEM-compatible keyboard models only (Tyros5, Genos, PSR-Sx7x).
BUT there are also many Custom Voices in Tyros3/Tyros4 format (.uvn/.uvd) available on the web for free.
P.S.
Tyros4 has only 128 Slots to load Custom Normal Voices (plus 20 Slots for Custom Drum Voices), and the installed Expansion Memory is used for both Custom Voices AND Premium Voices (of Premium Packs, downloadable from Yamaha MusicSoft shop).
Best regards,
Chris