Sounds nice but perhaps a best fit if you are 1) a small team 2) people know and trust each other already 3) people are all and equally comfortable asking for

Sounds nice but perhaps a best fit if you are 1) a small team 2) people know and trust each other already 3) people are all and equally comfortable asking for reviews (disturbing others, essentially). 4) the code has a priority on feature progress rather than stability and correctness. The situation can be very different in any of those aspects and mandatory review can still be a good idea. Are you building code that runs on surgery equipment together with 4 people you just met? I think you should probably have a formal review process. Or are you iterating towards an MVP for some webapp with a few people you consider friends? Lgtm just deploy to prod.