Os4 is not an offshoot of os3. It's a continued work from os3.
It's neither an offshoot nor a continued work.
"Exec SG was completely rewritten from scratch and from the ground up" (Ben Hermans, Hyperion, September 2002)
Now, "completely rewritten from scratch" can only be interpreted one way - that nothing, not a single line of code, remains from OS3.x (as if 68k assembly and BCPL would have been of any use anyway). There is nothing that makes OS4 any more of a continuation of OS3 than MorphOS or Aros other than the name.