Ok - dropping back to stock cameras.dat fixed at least part of the issue - the obs scope images now appear to be suitably smaller than the attack scope images - see screenshots below:
SH4 attack scope at 1.5X:
SH4 obs scope at 1.0X:
SH4 attack scope at 6X:
SH4 obs scope at 4X:
It's a start anyway
[edit] it's still exhibiting the oddity that in the attack scope the image looks bigger in SH4 (than in SH3) at 1.5X but smaller in SH4 (than in SH3) at 6X. In the obs scope, the image looks bigger in SH4 at 1X and at 4X. Very strange. Oh well - at least the obs scope is back to looking more "right" compared to the attack scope - guess I'll just have to forget how they used to look in SH3 and get used to how they look in SH4. After staring through those SH3 scopes for a couple of years, I was hoping I wouldn't have to recalibrate my eyes to be able to guesstimate ranges in SH4, but unless someone can tell me why changing the zoom numbers doesn't seem to work and how to make the ship images a little smaller (which, now that I think about it, doesn't sound like that great an idea anyway), looks like I'll have to just re-train my eyes.