We're tracking Rails 4 compatibility for the major gems we use, based on a bit of skimming GitHub Issues and Pull Requests. Here are the highlights from that list:

CanCan[1] has a '2.0' branch where Rails 4 support is being worked on. It also has PR #838 open for supporting strong_parameters in the current stable 1.6 tree.

Devise[2] pushed 3.0.0.rc about a month ago with Rails 4 support.

FriendlyId[3] seems to have work in progress on their master branch.

Acts-as-taggable-on[4] states in their README that they are compatible in version 2.4.1, but there seem to be a couple open issues related to Rails 4.

edit: formatting

[1]: https://github.com/ryanb/cancan/

[2]: https://github.com/plataformatec/devise/

[3]: https://github.com/FriendlyId/friendly_id

[4]: https://github.com/mbleigh/acts-as-taggable-on

I'm late to the party but we have a list of some outdated gems that are still not compatible with Rails 4 (we use a long-lived branch for rails 4 compatibility that should be merged once rails 4 is released and all our dependencies are compatible).

* client_side_validations : There is an outdated rails 4 branch

* client_side_validations-simple_form : this is trickier because it also needs compatibility with the next simple_form major version. AFAIK no work is done at the moment

* active_enum : looks like simply updating the gemspec may be enough

* ransack : there is an outdated rails-4 branch, I'm not sure what the current compatibility status is

Other than that a few gems have still not released stable versions with rails 4 compatibility and only have support in a RC or even just their master branch (for instance: simple_form, database_cleaner, sass-rails)

Active Admin is getting there in a branch, but I do like to have rubygems.org gems for production apps.

I've just started using the Turkee gem, and this needs the mass-assignment gem with Rails 4 to work correctly. Since I'm new to this one, I need to spend more time with it before I can send in some pull requests to provide patches

Compass Rails could use some help: https://github.com/Compass/compass-rails

There's also some issues with Ruby 2.0 support in it and in Compass itself, if memory serves...

Thanks! MongoMapper is at the top of my list. Hard dependency on ActiveModel 3.