SanDisk is now sampling the new 4GB microSDHC card to OEMs such as major phone manufacturers and mobile network operators (MNOs). Engineering samples are now available for evaluation, as mobile phone makers will need to upgrade the firmware in their handsets so that memory slots can support the new card.
It would seem like mobile network operators get first dibs on the 4gb cards. No word on other word from SanDisk other than to only say the 4GB unit will ship "later in 2007".
FYI, on 05/15/07 Samsung announced an 8GB MicroSD card but did not say when phone owners will be able to get their hands on it:
Samsung Press Release 8GB MicroSD card