Author of the PrusaMMU plugin here. The most recent release this month adds support for filament selection with the MMU on MK3.5/3.9/4.

There are two caveats, but I think they're reasonable concessions. First, you always need to slice your prints with the MMU profile, don't use the base MK4 profile. Second, when starting every print you will be prompted with the filament selector, if you want it to print as sliced(with the tools you picked) you can click the "Skip" button or let the prompt timeout.

Why the change in behavior?
Prusa removed the MMU Single profile in this version of their firmware. The MMU Single profile on the MK3 would send a `Tc` command to instruct the printer to prompt for a filament change. Octoprint would get the command and I could use it to prompt with the plugin. In the new "single" mode, they instruct you to slice with the base profile of your printer(without the MMU). The printer internally reads some comments in the code and uses that to handle the prompt before the print even starts, it also means that the MMU start code for single prints is not within the GCode but directly on the printer. So I couldn't reliably use that. There's a comment in their firmware that maybe they'll bring it back so fingers crossed.

I'm hoping to improve on it a bit and only prompt if there's a single tool in the gcode, I just need to find the time.

yes octoprint works with mmu, I just finished the mk3.5 upgrade and ran a print with 270 color changes, still works with no changes to octoprint, be sure to set extruders to 5 and check shared box.

Okay, got it. I'm still learning and wasn't fully aware of just what did the control work for any signals to the MMU. This is an easy enough configuration change to make!

Color/tool changes are in the g-code, not something Octoprint tells the printer specifically.

Thanks - that makes it quite clear to me what's going on: It's all in the g-code, and if OctoPrint is just passing the commands on to the printer, that makes sense.

The plugin only lets you see the information about MMU status. Once the developer can figure out how to hijack the starting procedure (this plugin isn't his full-time job, and makes no money on it at all), then it'll work like it has in the past where you can choose a tool to use for the print.

I have been reading through his work on this. The plugin still does not recognize my MMU3, but I may be one update behind at this point. I haven't been able to get back to it this week at all. I do see the testing discussions going on and one other person is using an MK3.5 (the problem seems to be with the 3.5 and 4). I did see that he was having an issue with recent changes in Prusa's handling of the setup. I'm following some of that, but since I haven't been involved from early on, I really don't have a complete handle on just what is going on. (And, of course, once he gets it working, there's always the chance that, at some point, Prusa will make more changes that will disable the plugin again!)

With the plugin working, you say it only lets me see the info. So I can't make changes with it within OctoPrint? That wouldn't be a big issue for me, since I prefer to do all that in the slicer, so if I have something special I can save the g-code or the slicer file and use it later, but I'm curious about that.

what do you use the plug-in for? is there a must have feature, thanks

I really hope I'm missing something, but with my limited experimentation, without the Prusa MMU plugin, I found OctoPrint could run the print, but did not do tool changes, since it didn't think the printer could. So it would work with an MMU (and no plugin) if all you want to do is print in one color.

As I said, I hope I'm missing something and that there's a way to tell OctoPrint, "Yes, this printer can do tool changes. You won't get what it's doing, but please just let it do them."

For now I'm using Prusa Connect, but there are features I get from OctoPrint plugins that I don't get from Prusa Link. Also, I'm using a webcam on OctoPrint. Looks like if I'm going to use a webcam with Prusa Connect, I need a Pi running a cam anyway, so I'd rather just be using OctoPrint for the whole thing. (For one thing, I also have an Ender 3 Pro - while Creality has taught me not to trust them and their machines, I still use it and it's easier to use one program for both printers.)

I would love to be able to just use OctoPrint for everything and to have the MMU plugin working. I've commented on the threads on the project on Github that I'll be glad to help with testing. It looks, though, like the developer hasn't been working on it for a short while -I'm not saying that as a complaint, just that it seems like it's been a few weeks since anything happened on there.

https://github.com/jukebox42/Octoprint-PrusaMMU/issues/43

https://github.com/jukebox42/Octoprint-PrusaMMU/pull/44

I haven't had a to of time to dig through the problem as I'm still working through the MMU issues I have with it loading. I'll get back to it though.

I don't have input for #2.