Congrats! Great achievement.
I would recommend to start working on top-down and impactful communication. This is a skillset that is arguably the most critical for your job, takes practice and time to hone. Everything else usually has a faster ramp up. Plus you have resources in McKinsey to help you (e.g., analytics team for excel, senior experts to get you smarter on industry, etc)
'The Pyramid Principle' is a great book to help you present your thinking clearly and effectively in pages/slides. It's written by Barbara Minto (ex-McKinsey Partner) and has helped many consultants make their discussions with clients/Partners more impact-oriented.
Apart from that, enjoy these 4 months. Travel if you can. Once you start McKinsey, you will be busy :)
Hope this helps!