Based on https://github.com/bcardarella/client_side_validations/issues/494 it looks like CSV will not support Rails 4 for a little bit longer.
However it also states
I don't see CSV 4.0 coming out for at least 2 months after Rails 4.0 lands (hopefully not that long) as there will be major changes on the JavaScript side of things. Because it will take so longer and people will want a Rails 4.0 compliant version of CSV suggesting they use master branch or perhaps releasing a alpha or pre version to partially satisfy them while we update might be the correct option.
So perhaps try pulling csv right from git? There is a 4.0 beta branch that might be worth trying...
Update
The CSV gem is no longer actively maintained. There are 3 different 4.0 branches on the repository, and the most recently updated of which is 4-0-useable (https://github.com/bcardarella/client_side_validations/branches/all).
Update 2
As @TheChamp says, the CSV gem now supports rails 4.2!