Alistair Crooks posted on the NetBSD announce mailing list the plans for the 5.0 branch.

Some highlights from the 4.0 branch changes:

+ during the development of NetBSD 5.0, the kernel version was bumped
73 times. The previous record was 29
+ there is a new kernel threading model which has better performance
than the previous implementation
+ we have introduced the file system journalling (WAPBL)
functionality, kindly donated by Wasabi Systems
+ much work has been done in the file system arena
+ the Xen port has updated to Xen 3.3, and has support for PAE domains
and amd64 domains (both dom0 and domU)
+ Xorg is now a part of our base system
+ providing multilib functionality (64bit platforms such as amd64 and
sparc64 can now compile and run 32bit binaries seamlessly)
+ almost every subsystem has been improved.
+ our contributed external software has moved to a new framework, so
as to make license issues clearer
+ many more device drivers are present in tree
+ all security-critical software is now compiled by default with stack
protection; this makes stack overflow and stack smashing attacks
more difficult to exploit
+ address space layout randomization is now supported on selected
platforms
+ NetBSD now supports creating position independent executables (PIE)
that can completely randomize the layout of stack, code, data, shared
library layout per executable invocation

Read the announcement.