Paul Graham in his essays reveals the secret of creative programming (and writing, and building start-ups) - build something that works and then iterate until it's good. Refactoring patterns of Agile Programming suggest the same. Fast Prototyping - once again. We know that the evolution is better than design. It's exactly what Integration is all about - evolving the system until it's fit for the Judgement Day - sorry - the System Testing.
So the Fourth Secret of Integration may be formulated as "The Integration is an Intelligent Evolution of (hopefully) Intelligently Designed Systems until they're fit to be Judged".