This has been a hot question lately. It is always asked anytime a future release of anything hits the news. Of course the answer to the question is nuanced based on the askers history. My default response is to always go with the stable and upgrade later. Especially if you new to the framework. By waiting it gives the framework time to stable out, documentation revised, third party packages updated, and community tutorials to be released.
Laravel 5 is considered beta until November and breaking changes will happen between now and then. Save yourself some pain and frustration by just playing around with v5 until then.
Update: Laravel 5 is now officially released and is the recommended version.