I’ve got both the Ultralite Mk3 Hybrid and now the Mk4.
I’ve had the Mk3 since 2011 and it has been a great interface on Mac via Firewire. My experience using it via USB was bad, lots of audio hiccups when processor usage became high on the host computer, or when other USB devices were under heavy load(lots of USB MIDI controllers and an external hard drive). Via Firewire, it was rock solid audio-wise.
The reason I upgraded to the mk4 is that since macOS Sierra, if I let my computer sleep while the interface is powered on and plugged in via Firewire, I will get a kernel panic. You can see mine and other reports of that here: http://www.motunation.com/forum/viewtopic.php?f=2&t=62567
I believe this issue is caused by changes in macOS with regard to Firewire and is unlikely to be fixed due to macOS dropping support of Firewire.
The Mk3 can be bus powered, but the Mk4 cannot(this was a surprise to me). The Mk4 has a MUCH brighter blue LED backed screen with no brightness control or ability to turn it off. The screen is beautiful, but very bright. The audio routing of the Mk4 is much more in depth than the Mk3. The Mk4 has a full routing matrix with no limitations. You can even output the entire stereo mix back into a channel and get digital feedback, which can be fun and useful if you want to create some weird effects. The Mk3 mixer channels can be controlled from the front panel with lots of knob twists and button presses, but the Mk4 is not granularly controllable from the front panel. You can only change the headphone and master volumes, choose mixer presets and control the volume of the front two XLR - 1/4" combo jacks via the front panel of the Mk4. The web UI for controlling the Mk4 is much nicer than CueMix on the Mk3, and I can control it via WIFI on an app from my phone.
The Mk4 is not very good at accepting a different sample rate than the one it is locked to. This results in a multi-second pause while the machine switches to the new sample rate. I never experienced this on the Mk3. Similarly, I’ve also experienced some issues where the sample rate change has caused the audio device to disappear from the Sound output menu on my Mac and I’ve had to reboot my computer and the device to get it back.
Another thing to look out for: There are two versions of the Ultralite Mk4. Before 2018 they were producing a version without the ESS Sabre 32 DAC, but as of the beginning of 2018 the box and product description has been updated to show that the machine has an ESS Sabre 32 DAC. I have the ESS Sabre 32 version. You can find reference to that change here: https://www.gearslutz.com/board/music-computers/1148240-motu-ultralite-mk4.html
I’ve seen reports of people getting old stock versions of the Mk4 which do not have the ESS Sabre 32 DAC, which is something to watch out for. The reports on reviews of the pre-ESS version of the Mk4 seem to be worse than the more recent reviews of what I assume are the new ESS versions.
Here’s the original box without the ESS DAC
Here’s the box I received with the ESS DAC sticker:
Anecdotally, I think the ESS DAC in the Mk4 sounds a lot better than whatever DAC was in the Mk3.
Overall, I love the Mk4 for its sound, price:features ratio, and the freedom of the routing matrix(something I always wanted on the Mk3), but I’ve felt like it has been quite a bit more fiddly than I’d like. I had some initial reliability problems with the Mk3 in 2011-2012 as well, which were eventually fixed by OS, firmware and driver updates, and I hope for the same with the Mk4.