Routing is an essential part of every MVC, Magento is not an exception to this rule. In order to understand how Magento routing works, it is necessary to get acquainted with execution flow that leads to certain request getting (or not getting) dispatched. It goes without saying that this knowledge comes in handy when you need to explain that 404 being thrown.
Since Magento CE 1.3.0 version, creating Magento admin controller required you to specify before="Mage_Adminhtml" or after="Mage_Adminhtml" attribute for your node at admin/routers/adminhtml/args/modules XML Path, depending on type of behavior you want to achieve. Like me, you probably have doubts on why this is necessary, and what is the background of this requirement? Simple answer would be that without it, it simply doesn't work. More precisely, Magento hits you right in the schnoz with 404 after you try accessing admin controller in question, if you don't set one of these attributes at your node. In this article I'll do my best to explain bug in Magento standard router causing this behavior, one that somehow managed to get packaged with last 25+ Magento CE versions without being noticed.