> You just have to handle the case of not having heap memory to spare

You have to successfully handle this case. Rust already handles this case by panicking, which you've unilaterally decided isn't safe for some reason. If that's not good enough you need to succeed. You must conjure the 4kB from nowhere. Show us all how it's done, or, and this might be the better choice, slink away to your fantasy land.

You are right at the end though, if you can't accept that running out of memory might happen what you do about that isn't dream up miraculous ways to "handle" it, you don't do dynamic allocations. Which of course works just fine in Rust.