And then he moved /boot to /usr ...

[Dale Carstensen]

Read the 4.0 man page for installboot.  The boot program that is place on
sector 1-whatever of your boot disk does have sector addresses for the
/boot program.  It is patched by the installboot program, which is not a
"dd" script as the 3.5 installboot program was.  In response to your later
message, this knowledge is probably makes you think that setup (I thought
it's name is now suninstall) is smart.