Consider yourself as a Project Manager and share best suited model in below scenarios $-3$: Your team is tasked with developing a new mobile application for a startup company. The project requirements are likely to evolve over time, and there is a need for flexibility and adaptability in the development process. Which project management model would you choose and why?

$4$: Your team is working on a project that involves complex and high$-$risk requirements, such as developing a new medical device. There is a need for rigorous risk management and regular validation of design assumptions. Which project management model would you choose and why?