Your framing of testing as risk reduction rather than confidence landed well, especially the line that an untested assumption is a gamble, and the question of what risk each test reduces. The layered model was the clearest part for me, with most incidents happening at integration boundaries and end to end tests protecting user trust rather than just covering code. The surgical debugging idea of reconstructing state before, execution trace, and state after also made failure feel traceable instead of random. Thanks for writing up Week 4 so plainly.