Spoiler alert: it will be sold off to satisfy the debts of the bankrupt corporation. That's SOP for bankruptcy court, and it would take an extremely persuasive case to convince the court to do otherwise.
It's one thing for Google etc to invest billions in cash and cloud services[1] into Anthropic with the understanding that money will be used for engineering and growth.
It's quite different for any buyer to come in and spend $4B on the secondary market for nothing but shares, with none of that money going to Anthropic.
There is a large appetite for exposure to the big AI companies, so maybe the demand is there. But it's not as simple as selling a large stock holding on the public market.
[1] https://www.axios.com/2023/10/30/google-invests-2-billion-an...
There are tons of big private equity funds and several trillion-dollar fund managers for which buying a 10% stake of a privately held 30 billion dollar startup would just be a question of some paperwork.