No that I'm aware of. What typically happens is that support for previous versions is dropped in favor of the newer version. Historically, however, security issues are backported to previous releases of Laravel.
This has been discussed quite a bit about the lack of an LTS release for Laravel and how off-putting it can be for business building with Laravel (and why Symfony tends to be preferred because of its LTS releases).
Someone else might be more knowledgeable than me on this, but there's my answer. You likely won't get anything more than critical security fixes in all versions of Laravel 4 once Laravel 5 is released.