There are similarities in our situation. I have a straight Nikon ED50 which, like your MM4 50, I find is "just a great combination of optical quality, size-&-weight and price," I also had a redundant Swarovski STS80 in the cupboard, and I was also interested in "rationalizing my set up". I wanted to see what a relatively lightweight 60mm scope with a modern zoom would add - maybe more brightness on dull days or towards the end of the day. So I bought an MM4 60 and the SDL v3 zoom. However, within 5 minutes of leaving the car park on IIRC the first time I used it I had heavy green fringes on the white belly of a gull on a mudbank. I am not a professional telescope tester, and I don't know how to provoke chromatic aberration. I had never experienced any CA in the STS80, ED50 or the ancient Zeiss 30x60 mirror scope I used before. This wasn't a conscious test of the scope - this was a viewing situation that had never caused a problem before in my 30+ years birding. IIRC the CA was there throughout the zoom range but it may have varied in magnitude. The body and lens went back to Opticron via the dealer for several months. Eventually they sent the zoom back with a new body (I checked the serial numbers !). Again within 5 minutes of leaving the car park I had a serious dark blue fringe on the white belly of a swimming Shellduck, and bizarrely for me, heavy blue fringes off its wake on the water. That example was returned for a full refund. Two bad examples in a row tells me this is either a bad batch, or normal business. However, there are many happy MM4 60 users on this forum who have never mentioned CA, but I note your comment that "the CA hasn't really been bothering me on the Opticron" so maybe it is normal business. Maybe the straight versions have a problem the more common angled versions don't have. IIRC someone on this forum went through 5 examples of a (Kowa ?) telescope before they found one they could live with. However, I don't have the time or the patience to go through Opticron's UK stock one-by-one to see if I can find a cherry. Why would I when I have an ED50 which is half the price, has a "great combination of optical quality, size-&-weight and price,"
and no CA
.
So my advice to you is try an MM4 60 and SDL v3 zoom for yourself - but only if you buy it (like I did) from an honest, no quibble, established dealer within easy reach - i.e. you can deal face-to-face and there's no messing with waiting in for couriers or packing and sending £800+ of delicate optics back by courier. I'd rather be birding than messing with that
.