If you don't already have Composer installed: Launch Git Bash, enter the following command: curl -s https://getcomposer.org/installer | php
This installs composer...
If you aren't already in your project folder,
cd to it.
Now we clone Illuminate (Laravel 4) and install it's dependencies with composer:
git clone git://github.com/illuminate/app.git && cd app && composer update
Now you are ready to test Laravel 4
Note: As it is under heavy develeopment you may want to fork it to keep track of its changes.