Warn if insufficient filament?  

RE: Warn if insufficient filament?

Your printer has no idea how much is left on your filament spool, and your slicer even less so. I don't know if Prusa connect can track filament consumption, that might be an option. 

The easiest thing is to get a cheap small scale (for up to 2 kg or so), they cost only a few Euros. Some vendors tell you the weight of an empty spool. If they don't just weigh the new spool and deduct the official filament weight (eg 1kg if you have a 1kg filament spool). Than you have the tara weight of the spool. If you have that you can weigh the spool anytime, even if the filament is inserted in the printer. Then you can find out anytime how much filament is left. 

With an Mk4, the filament sensor should detect if there is a filament runout and pause. One can manually exchange the filament then but there is the risk that one might see a seam if things are standing around very long. There is also a risk that the runout detection fails or the filament end gets stuck on the spool. 

If want to see how it works or test it, just cut off maybe a 1m of filament and start a small testprint, eg a Benchy. You can check then if the runout detection and pause works and also try out the manual filament exchange. Continue with the remaining spool afterwards and see how the Benchy turns out, especially if you can see the seam where you changed. 

RE: Warn if insufficient filament?

PrusaSlicer will tell you the weight of the filament it will need for the current job before you send it to your printer. If you use a filament that has a profile inside PrusaSlicer it will also tell you the weight including the weight of the spool. Now you'll just have to weigh your spool before starting the print. But if the filament runs out, the printer will stop, beep and demand that you re-insert new filament. After that the print will continue.
I don't know if an alert in Prusa Connect is possible, I am using OctoPrint and I am in the next room to the printer so I usually can hear the beeping.

RE:

Loading a new roll and carrying on works well. I had some problems on the MK3 with filament not ejecting properly, but the MK4 seems to do it better.

I use a rule of thumb for estimating the length of the remaining filament. The diameter of the filament on the spool is (to a crude approximation) 10cm. So each loop of filament on the spool is about pi times this in length, i.e. around 30cm. So three loops of filament is about a meter. If you are near the end of a roll, you can count how many loops of filament are left and so get the distance in meters, and then you can compare this to the value reported when you slice the model. It's not super accurate, but gives a rough idea of whether you need to pay attention or not.

RE: Warn if insufficient filament?

Also if you're using Octoprint or similar there's Spoolman https://github.com/Donkie/Spoolman/ that will track your spools and how much you have remaining.

RE: Warn if insufficient filament?

I thought the MK4 had this feature built-in of measuring the amount of filament left on a Prusament spool? At the very end of every Prusament spool there is a tiny little magnet and the Nextruder sends an electrical pulse through the filament every hour and calculates the resistance and through a complex algorithm gives you the remaining amount of filament left on the spool in meters or feet depending on which country you are in? And in turn notifies you prior to your print starting if you have sufficient filament for the job you want to print?

I think you just need to switch this feature on in the menu under EIM (extraordinary intuitive mode) in the settings? Good luck:)

RE: Warn if insufficient filament?

I don't believe this feature exists, 

the Mk4 has two types of filament sensor

for stand alone printers there is a tiny magnet, spring and ball bearing at the top of the extruder body, which is displaced relative to the integrated hall effect sensor when filament is present in the sensor hole. this movement sends an electronic signal to the processor and causes either a load filament action if filament is inserted, or a filament out indication, if the filament is removed / breaks. 
If the extruder motor loses steps, the printer can indicate a filament jam. 

for printers with MMU3, the ball bearing and spring are removed, and the magnet is inserted into the end of a Filament sensor lever, which is attached via linkages to the Idler lever of the extruder, 
When filament is inserted, it displaces the idler lever, which in turn moves the filament sensor lever, relative to the hall effect sensor in the extruder heatsink, again, signalling to the processor, as above. 

I am unaware of any tiny magnet in the filament rolls, there is however an RFID label inside the cardboard hub of the reel, which I Believe carries the unique roll identifier which is used for Inventry control and quality assurance of the roll of filament. 

Regards Joan

RE:

Filament runout detection should work well. The only big issue, which prusa can do nothing about, is that sometimes the end of the filament has a 90° hook that allows the manufacturer to hook it into a hole in the spool when winding it. If such a thing gets sucked into the extruder - good luck cleaning up the mess. 

Fortunately, when a spool runs low, you can usually just see how the manufacturer did it and act appropriately,  I.e. cut the filament before the hook goes in.