While we usually encourage using the most current/stable version of Kontakt Player, if you're using an older version of SIbelius, you may need to use an older version of Kontakt Player 4. While not publicly available, Native Instruments is permitting us to provide installers of Kontakt Player 4.2.4 for registered users of Virtual Drumline. Please fill out a support ticket to request a downloader for Kontakt Player 4.2.4.