I have both the Pixelcade LED and LCD and the upgrade from V2 to V4 has been a bit painful. Specifically:
1. LCD marquees for consoles (Atari 2600, Dreamcast, etc.) all disappeared, yet the LED marquees were fine. Turned out to be a naming issue and I ended up having to rename all of the ROMs to match the Pixelcade internal marquee names that I found on an Excel spreadsheet. This all worked fine with V2 so it was odd that this happened.
2. Since the upgrade, the both the LCD and LED occasionally freeze up unexpectedly and require me to unplug and re-plug in the devices. Even power cycling the cab does not fix it. It is seemingly random and this never happened in V2.
3. I have a USB flash drive plugged into the LCD and in Windows Explorer, I can access both the USB flash drive and the internal Pixelcade storage. Suddenly without warning, the USB drive disappeared. I could still access the Pixelcade internal storage via the IP address and host name, but the USB drive was gone. This was the case for a few days, when it suddenly re-appeared and has been fine since then.
4. The Pixelcade upgrade overwrites an existing LED Blinky configuration without warning (or if there was a warning, I missed it). I didn't remember exactly what my settings were so I had to play around a bit to get it back to the way it was prior to the upgrade. There is a bit of a lag with LED Blinky integration but I know there are some settings in LED Blinky that can be changed to address that, but I just haven't gotten to that yet.
5. My only outstanding question is how to I access the LED config to set the default marquee? The URL that I previously had for LED V2 config now points to the Pixelcade companion, which only seems to be applicable to the LCD.
Despite these issues, the upgrade did solve some outstanding problems with V2 where the LED marquees would not always display and also an issue where similarly named marquees would not display correctly (Ready 2 Rumble would display for both Ready 2 Rumble and Ready 2 Rumble 2, for example).
On #1, I did send an email earlier to do a remote session there, I'd need to see the logs to see what is going on there. https://pixelcade.org/lcdartwork/
pixelcade companion in your case is just for LCD. To set the default marquee on LED, here's how to do that https://pixelcade.org/customizing/