Interesting. Seems to be at slightly higher revs too, about 5k for the 45bhp engine at 5.3k for the 4X4 which would point to better breathing which could be the different inlet cam, coupled with a different ignition curve?
I mentioned a Cat because maybe the non 4X4 may have one, but then realsied the LX I just sold didn't have a Cat either and that was a 45bhp car so that's not the difference.
Are there differences in the carb? Does the 4X4 have a twin-choke carb? That would allow for more power higher up the rev range.