The move is most credible when you can explain what you have already learned at Big 4 and what you are specifically seeking next. Avoid framing MBB as simply more prestigious.

Good reasons can include broader exposure to CEO-level questions, different project types, a more generalist development path, or a platform that fits your long-term goals. The exact answer should match your current experience.

Your application also needs to show that the move is additive. Translate your Big 4 work into ownership, problem solving, client impact, and measurable results rather than assuming the firm name speaks for itself. If useful, this guide on MBB vs. Big 4 gives more detail.

I made mistake of saying bigger clients + prestige. did not land well.

talk about what is actually different in the work / development, not just the logo.

Big 4 strategy is already relevant experience. I would make the story about what you're adding next.