Just want to add my experience here...

My recommendation to anyone else getting this issue, is call up tech support (when the phone system asks for your virgin media account number and phone number, just spam digits and it'll take you straight through to the main menu, then to a real person, otherwise you have to wait 24 hours for the automated system to detect a fault before you can speak to a real person), ask to speak to cancellations, and ask for them to arrange an engineer callout. Explain the issue to the engineer, and maybe even show them this thread, and if they don't have a Hub 4 in their van, ask them to leave a note on the system recommending you're posted a hub 4. Then call up tech support, and explain to them that the engineer recommended you get a Hub 4. If they tell you the hub 4 isn't compatible in your area, ask to speak to cancellations, explain the situation to them, and ask them to leave another note on the system. Then ask them to transfer you back to technical support, explain the issue again, and repeat until they order one for you... Be prepared to spend literally hours going in circles with very unhelpful tech support agents. The hub 4 is compatible on any Virgin package, not just gigabit, don't let them fool you.

I had almost the exact same issue, with my Superhub 3 restarting itself multiple times per day, for no apparent reason... Main fixes tried by Virgin and myself were:
-Replacement Superhub 3 (tried twice)
-Changed the connector on the cable going into the back of the superhub
-Provided me with a direct line going to my house, as I had been getting my connection from my neighbour's box on the outside of the house
-Put me on a different "tap" on the green box at the end of the road (not sure if tap is a technical term, I assume it just means port)
-Powered the superhub via a UPS in case there was a problem with my mains
-Changed the ethernet cable I was using with the superhub
-Put the superhub in Modem only mode, and used it with a third party router

After all this, the issue was exactly the same. Hub just appeared to restart (at least, it showed all the flashing lights associated with a restart).

After reading this thread, I realised that the hub was only rebooting while I was connected to my work's VPN. We use Watchguard Mobile VPN with SSL client. I do have Nord VPN, but I was not using it at the times of the restarts.

After an insane amount of back and forth between technical support, the cancellations department, and in-person engineers, they finally agreed to post me a hub 4, which arrived on the 12th. The tech support repeatedly told me that the superhub 4 was only compatible with gigabit packages, which isn't available in my area, but the in-person engineers told me that wasn't true, but they never had one in their van.

Now, it is early days yet, but I can say with a reasonably degree of certainty that my issue now appears to be resolved. I have as yet not had a single reboot after 2 full days of using the VPN.

Absolutely bizarre that even in Modem only mode, there seems to be a bug with the superhub 3 with certain VPN traffic. But, God-willing, it seems the same issue isn't present in the Hub 4, so I'll take it.