Looks like there is more to be done for Rails 4.
Inside you Gemfile add.
gem 'bootstrap-sass', '~> 3.2.0.2'
Next you need to run
$bundle install
This is the part no one has mentioned
Open you config/application.rb file
add this right before the second to last end
config.assets.precompile += %w(*.png *.jpg *.jpeg *.gif)
as seen here about a 20% down the page.
Next create a custom.css.scss file in this directory
app/assets/stylesheets
as seen here a little further down from the above task
Inside that file include
@import "bootstrap";
Now stop your server and restart and everything should work fine.
I tried to run bootstrap without using a gem but it didn't work for me.
Hope that helps someone out!