I'm not looking for support for new OS features. I'm just thinking of
support for when a new OS actually completely breaks an SB4-based
installer. I think this last happened with Windows 2000 SP1 -- the
installers were incorrectly reporting it as an unsupported operating
system (and refusing to permit installation) until you released a fix
for that. If something like that were to happen, I'd want a quick fix.
SB4 is only one version back, so I don't think that's unreasnable.

No I haven't checked 5.1 yet. Given that my SB4-based installer works,
and I have lots of other tasks on my plate, spending time porting to
SB5 isn't high on my list.