For others following along, I would like to point out that my original conclusion was that 1.4 would be required given the explicit statement in the document:

  To build Go 1.x, for x ≥ 5, it will be necessary to have
  Go 1.4 installed already, in $GOROOT_BOOTSTRAP.

This is subtly different from the statement in the new ports section of the document:

  For Go 1.x (x ≥ 5), new ports will have to be done by
  cross-compiling test binaries on a working system, 
  copying the binaries over to the target, and running and 
  debugging them there.