I finally got 3 working yesterday. I upgraded to 4 this afternoon. The upgrade to 4 was uneventful. So far all is quiet on the Western Front.
After being down for a couple of days v4 restored everything for me.
I just tried the direst update from V2 to V4 and now HA is dead for me on a reboot. I can get into the CLI but HA does not start.
Maybe I should have removed the integration first before the update or remove the addon completely before adding it back in.
I tried removing Mosquito from cli with
hassio addons uninstall --name “core_mosquitto”
but it still wouldn’t come back up so I am currently flashing Hassio to the card and hope to get it back up from a backup with V2 running soon.
UPDATE
It appears that an issue with Chromecast was stopping it coming back up. I have tried again but, as mentioned before, you cannot use “homeassistant” as the username. I changed it and changed 1 Sonoff and that works. Not ideal as some of my devices are standalone with no working OTA option so will need to be done manually 
I made sure that the integration was removed before restarting HA then adding it under back to the “configured” section.
Try ssh root to your pi, go to /config/.storage and delete the file core.config_entries, then go into /config and edit out all mqtt references in your configuration.yaml
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From what you’ve said, as my mqtt username isn’t homeassistant, I should be able to update the add-on with no further action?
Yes you should be good to go but make a Snap shot first and copy it off your device.
Remove the integration, restart HA, Update Mosquito to v4, restart HA then add the integration back. This is what I did to test my Sonoff and Espeasy Wemos D1. I have several Bruh NodeMCU’s and Wemos D1’s that seem to look up when I flash them OTA. I really need to get most of them running ESPeasy so that I have a web interface to change this in the future.
Good luck and I am keeping everything crossed for you - This has taken me most of the day to get back and working as it should.
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Cheers, because mines a VM I cloned the whole bloody thing first 
Anyway, stopped the add-on, updated and restarted, everything just like it was before, still not using auth.
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Good to hear that its working for you.
Just making a list of all the MQTT devices I have - currently found 5 that will need to be manually update via IDE from a laptop - leaving that for another day 
Same question…Anyone from v2->v4?
Lol, you mean apart from the two of us above you 
I was really feeling brave this evening (and in the meantime I was really experienced in restoring backups) and the update from V2 to V4 went flawless…didn’t have to change anything in my setup (I am using separate accounts for my Tasmota devices). Just used the accounts in the addon, so no separately created accounts in Hassio.
One thing I noticed is the authentication process if using a HA account can be slow. I had devices that were timing out when connecting, even though Mosquito showed authentication was successful. My solution was to increase the timeout on the devices from 1 second to 5 seconds and now they are connecting successfully.
Where did you set the timeout?
I made the initial mistake of upgrading from v2 to v3. Everything went tits-up, so I reverted back to v2 using a previously saved snapshot.
Today I upgraded from v2 to v4 directly, and had no issues. I had previously removed the mqtt entry in configuration.yaml and set it up via integrations. My supervisor is on version 139, and the host system on my rPi3 is running HassOS 1.12.
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The devices run custom code so the change is in the code for the device.
Actually i still cannote run all of my mqtt services.
1541904058: ACL denying access to client with dangerous client id “/OpenMQTTGateway”
How can i disable this ACL?
Tried everything suggested here but still cannot connect. Is there a way to install V2 without a backup image?
You still helped…your post made me look at my wifi power setting for my esp. Changed that from lite none, and now i no longer have constant connect disconnects.
My previous settings had worked on v2,
I had to re-flash all my esp devices with an updated PubSubClient.h. Had to change the MQTT_KEEPALIVE from 15 to 60 and this fixed my issues with this problem.
The line that needs to be updated is on line 33. #define MQTT_KEEPALIVE 60
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