That cryptic statement was the latest news from Christopher Paolini in the March 2009 Alagaesia.com newsletter, but what could it mean?
At the point of writing, Paolini had worked on the book since October 2008 and he typically takes three years to finish a book and reach the point of publication. We can estimate that he had taken six months to reach this section. It is stated that this book is the longest yet and the book with the most chapters at present is Eldest with 71. If we assume that the fourth book has 84 chapters, we may estimate that he was one sixth of the way through in March 2009, around chapter 14.
It was also stated that in the fourth book we would see an elven death and burial ceremony. So, it may be assumed that an elf dies in a duel. "Elf" is also in the plural, as is "werecat". Would tensions between Varden factions flare? If so, it seems likely that the elves most likely to be in proximity to the werecats are Blödhgarm and his spellcasters.
Which werecat? Solembum often accompanies Angela, who King Grimrr Halfpaw is seen to distrust in the fourth chapter. Evidently, a spear is involved so this may be Solembum using Angela's Hûthvír instead of his usual dagger.
If Solembum killed Blödhgarm, then Ildrid the Beautiful or (more likely) Liotha may be driven to avenge him by attacking either Solembum or another member of the Varden. A "knife in the back" was mentioned in the May 2010 newsletter when it could be presumed that Paolini was three quarters of the way through the book or around chapter 63. I will leave the rest to speculation.