Ok, last night I decided to make sure and rule out everything on the system side. I cleaned out all the grease around the cd laser gears and tracks and whatnot with rubbing alcohol, and replaced it with Molykote 44 Light. After this, Rondo still worked perfectly... and the 4-in-1 wouldn't load at all
Then I tweaked all the pots according to this guide:
http://www.pcenginefx.com/forums/index.php?topic=8926.msg152736#msg152736After that, the laser seemed to operate more quietly, and the carriage moved around to find different tracks even quicker and more efficiently in the music test on Rondo... but the 4-in-1 still wouldn't load
Finally, I decided I'd try the pot right on the side of the laser. I turned it about 45 degrees clockwise, tried loading the 4-in-1 one more time, and to my delight, it started instantly.
It seems to be working perfectly now; all the problems areas where it would stall, crackle, or fail to load outright, have cleared up. In the Gate of Thunder music test, the last track is on the very ragged edge of the disc, and it loads up super quick and plays rock solid.
Of course, everything in Rondo still works great too. I still don't know why it never had much problem and the 4-in-1 was so bad though. Granted, the Rondo disc is mint, but the 4-in-1 really only has a few light scratches. Maybe there's something different about US and Japanese Turbo CD's, I don't know.
Since everything seems to be good now, I'm going to leave well enough alone. I'll keep the CD cleaning tips in mind for the future though.