RE:
OctoPrint just works. I simply changed the USB cables (MK4 needs USB-C), plugged it in, and everything worked straight off. OctoPrint uses the same printer profile as before, but there are slight differences that are documented in https://help.prusa3d.com/guide/octoprint-setup-on-mk4-mk3-9-xl_646395 .
So just for the fun of it, I'm doing a 0.05mm ultradetail print of 3DBenchy, which is going to take 6½ hours. I may try and do a 12 minute 3DBenchy afterwards for comparison.
PrusaLink simply says that the printer is busy, though the xLED shows a friendly green octopus; tune, pause and stop controls; and the current temperatures. The xLED is a dream after its predecessor: it's a pity it's not a touch-screen, but it works well enough that is soon forgotten.