Playing Devil's advocate for maybe the first time ever on PPRUNE and obeying the OP who asks for a discussion on ASR(f) only - As someone who regularly does EU flights, it is still 4 crew. And since we dont want to be chatting about experience levels, training ban, 5 years to upgrade etc etc, there is no difference.
Long haul is still difficult and everyone is fubar on landing. All FOs that I fly with (who strangely enough don't have applications in with Qantas or BA) say there is no difference.
And I'm pretty sure our Freighter cousins would love a 2nd SO on some trips.
Fatigue is Fatigue and should always be ASRf'd, 2 SOs or not.
Now if we are allowed to talk about the wider matters that Having only 1 FO on the flight deck would cause after a 12 hour flight, typhoon in HKG divert to TPE - then that's a totally different discussion.