Please check the following image - http://www.sitefinity.com/Libraries/Features_Screenshots/Sitefinity_Lifecycle.sflb.ashx
it shows the lifecycle of the 3.x and 4.x versions. This is published in our FAQ: http://www.sitefinity.com/purchase/purchase-faq.aspx under Existing 3.x Customers.
With version 4.2, we will introduce Fluent API for Definitions - basically the building elements of all controls and custom modules. This API will greatly simplify the module creation.