Mongoid 4 on Rails 3 compatability

Question

Some of the new features in Mongoid 4 (4.0.0.alpha) are very useful to me, in particular, aliased attributes working in criteria.

For the time being, however, I'm stuck on Rails 3.2.

When I try to bundle update mongoid after specifying 4.0.0.alpha, I encounter compatability issues on the activemodel gem:

mongoid (= 4.0.0.alpha1) ruby depends on
  activemodel (~> 4.0.0) ruby

rails (= 3.2.13) ruby depends on
  activemodel (3.2.13)

Is there anyway to use Mongoid 4 with Rails 3?