No, not as written. conceptually, this already happens in a different way. but doing this wouldn't mitigate the issues caused by Meltdown and Spectre.
JavaScript executing in browser javascript engines already live in their own isolated environments in ring 3, so effectively ring 4 if you'd like, but these are just conceptual rings so all unprivileged operations are in ring 3, JS engines (and other things) are just designed to be isolated from other code running in ring 3.
Microsoft has gone a step further and has something called "Windows Defender Application Guard" that effectively runs MS Edge in a separate isolated VM, which might be along the lines of your thinking.[1]
The problem is that any application in ring 3 down can exploit Meltdown and Spectre, so even code running in Application Guard can still potentially read privileged memory anywhere on the system, so even this is not effective at combatting the problem.
It's not practical to make unprivileged code currently running in ring 3 more trusted and doing so would require much worse changes. You might conceptually have a completely separate system on system architecture for the purpose of running completely untrusted code, but this is overkill, as the current architecture is supposed to be just this. But anything along these lines is still hypothetical and would require hardware changes that can't be patched in.
[1] Intent of MS of doing so is a bit convoluted, as they seem to be saying that they don't trust their own browser engine to be fully sandboxed, but they don't seem to provide why they feel this, nor why they feel a VM wouldn't potentially have it's own escapes, such as those demoed here https://arstechnica.com/information-technology/2017/03/hack-... so this is probably really just marketing exploiting the fears of business users in order to sell enterprise licenses.