nilrt.conf: bump DISTRO_VERSION to 4.0
nilrt.conf: bump DISTRO_VERSION to 4.0

Version 4.0 is already reserved for this release, so make the switch.

Signed-off-by: Ken Sharp <e6c8ad8f54d15df4645e8aaf3f7a209bc3688570@ni.com>

ni/meta-nilrt,ni/meta-nilrt,ni/meta-nilrt,ni/meta-nilrt,ni/meta-nilrt