How to use newer Mono? (MonoBleedingEdge?)

I’m using Unity 4.3.3 and would like to use .NET 4.0 code, which would match Mono 2.8.
Is there any way to make this possible? I realized that there is a MonoBleedingEdge folder in the install, which would support Mono 2.10, apparently (using /bin mono.exe -V).
I tried the naive way, copying the content of the MonoBleedingEdge folder into the mono folder, but that didn’t work at all.
I know that there are multiple threads on the mono topic, but there are different answers (like: Mono 2.6 is used because of licensing problems, or: Mono 2.8 is used, because upgrading Mono breaks Unity).

What Mono version is planned for Unity 5 (and when will this be shipped? Is there a roadmap?)?

The unity mono runtime is a custom 2.6.5 which is basically .net 3.5 with some changes.
There’s a table of the implemented features per platform somewhere.

The runtime simply doesn’t understand new language features in 4.0 code. This is where some of the hard work goes in. They need to think about every feature that gets into the runtime, as it increases the runtime size and has to work with a good performance on all platforms.

So, there’s no way to trick this into working till it’s actually supported. If you found something from the next version they’re working on left in, it still might be incomplete or needing extra steps to make it work and surely not production ready, so using it is not recommended. The final next version may not just add features compared to the version you found, but also remove some again that deemed unfit.