Even an hour-long impromptu talk is a lot to ask for a job interview. I can tell in 5 or 10 minutes if you're a confident speaker who is aiming at the right level. And since you could be asked to do any topic, you basically have to write, prepare, and rehearse the whole 4 hour thing, just in case. That's a lot of work. I've interviewed technical trainers before and we told them to prepare a 10 minute talk on something technical of their choice, and during the interview we asked for 5 minutes completely impromptu (tell us how to get here from your place.) These showed their pacing, voice control, detail level, ability to stay on track, etc. No need for an hour of talk by them or of listening by us.
If I were being asked to do 10 or 15 minutes on literally any part of the guide, to be chosen on the spot and with perhaps a minute to prepare, here's what I would do.
First, I would spend about half an hour to an hour skimming the guide to establish the overall theme of this device, which might be one or more of:
- it is very advanced compared to all the competitors
- it has a ton of settings and knobs
- it has a ton of lights and sounds
- it is very delicate and should not be bumped
- it is very robust and sturdy
- it is small and compact
- it is very intuitive to use
Then I would learn simple basics like how to turn it on and off, how to configure it for metric/imperial or adult/pediatric or other commonish things. Going through the instructions I would connect them in my head to the themes. So it's not just "press the X button" it's "because on this device the most useful things are just one button press away, simply press the X button". Or alternatively "because on this device everything is on the central touch screen, simply touch A then on the next dialog press the X button." Making it sound like the most natural and obvious way to do the thing.
Since you can't memorize 4 hours of training, I would not try, but I would read the guide repeatedly so I understood how to work the device and the sorts of things people need to do. To use the analogy of a car, could you do an hour on "how to check the fluids under the hood" for an arbitrary car? You could if you remember already what "the fluids" are and if you're allowed 30 seconds to remind yourself where the dipstick, washer fluid reservoir etc are, as well as trickier things like the radiator cap. (Or if you can fake it by telling the interviewer "I would have a diagram here and would point out the location of A, B, and C as well as the special instructions for opening the A." This can be very convincing and accurately reflects how you would prepare training material. You could even "point out" these locations on an entirely imaginary diagram that is not actually projected behind you.)
Between knowing the general principles that the guide covers, knowing the "theme" you're going to hang your instruction on, and being familiar enough with the guide that a quick glance will refresh the details you need, it might be possible to prepare for this task. Might. At a 5-10 minute length, it would be far more feasible. But obviously some people have agreed to the hour, so you probably must to get the job.