The composer simplifies and unifies the distribution of code in PHP, there is not much difference, however bundles for Laravel 3 are mostly not compatible with Laravel 4.
By composer, each package you want to install is a dependency of the project, so by editing the composer.json file, you must add the package in question in the require session, to search the packages, you can use packagist.org
Here's an example, there is a Generator Package for Laravel 4, made by Jeffrey Way, if I want to use it in the project, I should add the package to composer.json in the require
"laravel/framework": "4.0.*" already existed, I added the line "way / generators": "dev-master"
This still does not do anything in the project, so that the library is downloaded, run the command at the root of the project:
php composer.phar update
Or if the composer is installed Globally in your environment
Once this is done, the package source will be available in the vendor folder, and it will be loaded automatically by the composer autoloader, you do not need to use include or require.
Each package can be used separately, or as in the case of the "Generators" package in the example, it integrates with Laravel 4, simply add this line:
to the app / config / app.php file in the session providers.
Before trying to understand the Packages for Laravel 4, try to study Composer a little bit more, it is a phenomenal and indispensable tool for PHP programmers.
I hope I have been clear.