Were it by any chance python code errors? and happening the moment you tried to configure new VM's, network, etc. during the last step after first boot?
In which case, you can solve it either by identifying missing UEFI/BIOS settings, updating UEFI/BIOS (be sure you know what you do first, so you don't brick your hardware here), and lastly but not least, you can pull out the HDD/SSD/NMMe, whichever you use, and then put it in another computer. Install Qubes, fully update Qubes, and then pull the drive back out and put it back in your original machine. I've successfully done this a few times to bypass the Qubes 4 errors and other installation issues that normally are fixed with updates, but are not fixed in the install medium. This in particular helps in Qubes 4, where this step frequently happens during the last setup step on first boot on the Qubes 4 RC-1 / RC-2 and RC-3. Hopefully RC-4 is free of this issue.
Precausions!!
1): Using this method, it's best to only install with Grub, unless you know your way around UEFI/EFI install paths. If you use UEFI/EFI, then not only will it be tricky to regain any old EFI paths you had on the other machine, it will also be tricky to install on the current machine when you put it back in. If you on the other hand install with LegacyBIOS/Grub, then it should just work, plain and simple.
2: Might want to pull out any drives you got on the other machine, while installing Qubes, just in case anything should be written to the drives, like new grub configurations, etc. just to be safe, remove them. Then put them back in as they were when you pull the drive out you just installed Qubes on (best use same cables if any software is sensitive to the drive numbers).
3: When you put it back in the old machine, and errors that were corrected between RC-3 and now today, will be fixed.
I've had quite a lot of success on multiple different Qubes 4 machines using this method as of late.
Maybe worth a a try if you got a sparing computer around that can run Qubes?
Just keep in mind your Qubes will be exposed to unsafe hardware if the other machine isn't trustworthy. But for the most part, if its beween not getting Qubes to work, and this, then it's well worth the risk imho. Also the risk might be low, depending on what you pulled that machine through, and/or your public profile. Like probably only high profile people are heavily targeted, like really heavy profiles. You're probably not at much rish in this day and age, but you never know for sure though, but probably not big odds.
Either way, it's definitely worth it if its your last option to try.
Also your description of the issue sounds a lot like the issues I encounter, where it helps installing on another machine, update, and then put the drive back. It may be worth a shot if you got the means to do it.
Further keep in mind if you break any still valid warrenties, and/or be sure you don't do something that might break your computer. Think twice, or triple, and ask if you got questions before doing something you're not sure about is doing. Just to be safer, there is always a risk when going into UEFI/BIOS settings, nevermind updating it which can brick your system if it breaks halffway or there is a corruption in your downloaded update file, poweroutages during UEFI/BIOS update, instability causing freezes, etc. etc.
Glad you love it already, you will likeliy only grow even happier with Qubes over time :-) It's an amazing project indeed! and there is still a lot of new things going on, expanding the idea of what Qubes is about further. Like the recent news here https://www.qubes-os.org/news/2018/01/22/qubes-air/ as far as I understood it it's not changing Qubes or the ideas and principles behind Qubes, but it is however expanding what you can opt-in and with choices you have with Qubes.
Ad for changing updates over Tor, yes, and it's super easy too :-)
Go to Qubes menu ---> System Tools ---> Global Qubes Settings. In there, change the UpdateVM away from sys-whonix, and put it to sys-firewall. There are many in the list, but on any default install, you can only use sys-whonix or sys-firewall, the former for Tor, and the latter for regular network.
If you expand your default setup to include an VPN connection, multiple of VPN connections, or multiple of Tor connections, combine, or have multiple of firewalls, then here too is where you'll change where Qubes gets its updates from :-)
Note in Qubes 4, Qubes will handle the default pre-installed template updates, so that templates has no internet themselves (unlike Qubes 3.2. where every template has internet on their own). You still update the same way, but Qubes will handle the distribution of template packages. As far as I know, this too will follow this setting, although I'm only almost sure, not entire, I did not yet check if my assumption here is true or not.