Using RELENG_8 to compile for older RELENG_x

[grarpamp]

I'm on RELENG_8, works great. I've been bugged to compile some
things for RELENG_4 boxes. Due to administrative fiat, I have to
compile externally and ship them the results, no login.

So in general, how do I use my RELENG_8 boxes to compile apps that
will run on RELENG_4?

Similarly, how can I make buildworld/buildkernel/release for RELENG_4
on RELENG_8?

I have RELENG_4 sources on disk, but of course things don't compile/run
right. Going forward is easy, I've just no idea how to go backwards :)