Phase 4 is what we call Iterative Execution.
Finally, Phase 4 uses a combination of Scrum, Kanban, and Agile engineering practices to iteratively deliver the remainder of the solution. The goal is to build quality into the solution throughout so that the product, while incomplete from a capability perspective, is, from a quality perspective, release-ready throughout execution.
No matter how disciplined leaders are in the early phases, discovery still happens during Phase 4—product development is inherently complex work. So, surfacing learnings and adapting the plan is an ongoing activity.
Ideally, these discoveries are minor because the major complexity was resolved in Phase 2. But sometimes complexity is outside our control or even visibility. Competitors, partners, markets, and regulators can all do unexpected things. We need to remain agile, even as we work towards predictability.