Guys, thanks a million for all the help so far!
Going to be some close scrutiny of all available images and info soon, closest match to photos will take the prize. Please continue with your opinions on this.
Re the fueslage '4', I suspect both Wojtek and Wayne are right: seems to me to be a very slight curve to it as Wayne said, but appears more pronounced than possibly is due to the slant of the upper stroke on the curved fueslage side, as Wojtek said.
I'll try to track pics of other aircraft from this unit to compare styles of numbering.
The upper stroke is certainly longer than appears in the profiles in any case, and not just due to foreshortening.
Research, research, research...