It's definitely the tranny leaking. It's gearbox oil. The leak appears to be coming from where the case mates with the rear cover, at the gasket. Oil forms at the two bottom bolts and drips from there. Below is a picture of where the drips seem to be coming from, circled in red.
NB: that's not my picture and not my gearbox. It's not even a 4-speed. It's just to give a rough idea of where the leak seems to be coming from.
About a year ago I pulled the gearbox apart myself for a gearbox tech day. I replaced all seals and bearings and put it back together. The bike was not running when I purchased it, so I can't say for certain whether it had the leak before I pulled it apart. After installing it and filling it with oil I noticed the leak. I tried heating the rear cover and gently tapping to see if it wasn't seated properly and re-torqued the nuts to spec. No joy.
I took it to my neighbor Brent at Brent's Motor Works and he inspected the mating surfaces, put on a clean and dry gasked, and re-shimmed it to make sure overshimming wasn't the issue. No joy.
I'm running out of ideas. My final guess before scrapping it is that the rear cover got warped when I was pulling it apart. I picked up a new rear cover from eBay. I'm going to try swapping it with the old cover and see if that fixes it. If not, I'll probably try a whole new case. I have all the tools, except a depth mic and a shim plate for re-shimming.