How to create distribution for later NFS sysinstall on other box?

Some of my systems are 5.1-CURRENT but I still have some older 4.x
boxes.  I'd like to upgrade them to the same OS as my 5.1 boxes.

It seems stupid to feed them boot floppies then FTP the OS across the
WAN from freebsd.org or mirrors.  

I expect there's a way to build a distribution on my main 5.1 system
then use sysinstall on the target 4.x to install via NFS (or FTP
or...) over the LAN.  I have not found any pointers on doing this in
the Handbook or a couple quick Googles (perhaps I'm searching on the
wrong terms).  Seems it should be something like this on the server:

      cd /usr/src
      make distribution

I'd like to make the distribution based on my 5.1-CURRENT, rather than
copying/creating a 5.1-RELEASE image so I won't have to do a
subsequent update to get it CURRENT.

Any pointers? If I'm missing obvious docs, just tell me where to RTFM. :-)

Thanks.