Start a new edge Rails 4.0 project

OK here's what worked for me...

I use rbenv - so I set up a rbenv gemset for my new project...

Next I did gem install bundler.

Then I created a Gemfile inside my project folder like so:

source 'https://rubygems.org'
ruby '1.9.3'

gem 'rails', github: 'rails/rails'

Finally I ran bundle.

Now I've got the rails command available in my gemset so I can run bundle exec rails new . and I'm off to the races!