Ah, I see: yes, I can't tell if that would still be the case. Just read through that article, very interesting.
It may be worth posting a follow-up question to "onthenet" in that TBB thread - to check if they are using their 4G router in "4G Only" mode, or if it's set to "3G/4G Auto" (or just "Auto" - i.e. 2G or 3G or 4G). Most 4G router users in my experience do set their routers to a network preference mode of "4G only", which will ensure it stays on a 4G connection - otherwise, if it finds a stronger 3G signal, it may switch to that (with a general decrease in speeds).
I suppose one other option would be to find another cheap fixed line broadband contract, that's fast enough for you but as cheap as possible, which would provide Static IP address as usual, and just combine that (via Load Balancing) with a Three 4G link too, for extra speed, flexibility and failover options - just a thought. It all depends on your particular requirements of course.
Anyway - yes, in your case it may be best to do a bit more digging, otherwise just wait for further developments - hopefully IPv6 will be rolled-out soon. It will also be interesting to see how the upcoming 5G handles this, I would like to think that they would go with IPv6 from the start with that, surely? Will be interesting to follow that over the next 2 years or so.