If anybody does know (and I'm sure some higher ups in VM do) then they will not be allowed to discuss it in public until the box is officially launched.
I'd be very surprised if all current Tivo subscribers got the new box for no extra cost. It might be offered to customers on the VIP package at no extra cost, but I suspect all other package customers will have to pay something extra for it. Sky are charging setup fees for existing customers to upgrade to their new 4k box so I'd expect VM to do the same, at least until they decide to drop the old Tivo box and make the new box the standard which I doubt they will be doing anytime soon.
There is some 4K content from BT Sport & Sky Sport, but I think most of the currently available content is streamed from Amazon Prime, Netflix & YouTube.
Disclaimer - I don't work for Virgin Media. I'm just another VM user trying to help out so my answers may be wrong If my answer solves your problem please mark it as helpful as it may help others My setup: Vivid 200 Optical fibre with Superhub 2 in modem mode connected to a Netgear R7000 router. Telewest/VM user since 2001.