Great question @NaimK! The first thing I would say is that it is never worth being too prescriptive with these things - if one section is longer than another then that is not a problem
It is probably good to think about how you are roughly going to apportion each section though, as you have been doing. I think that the breakdown they provide you with actually closely mirrors the standard cover letter structure (1. Why law? 2. Why this firm? 3. Why you?) although the order is different and they have been slightly more prescriptive about what you should include in the 'Why you?' section.
As a result, I think that you could perhaps split your answer into thirds, with points 3 and 4 together combining to form around a third of your answer, or a little more, given that the single achievement of importance will probably not take up too much space. How about ~400 words for points 1 and 2 combined and ~250 words for 3 and 4 combined? Again, it does not matter if you veer away from this at all, but I generally agree with you that it is good to write in depth about your motivations!
Much appreciated, James. I was thinking that the extra curricular could be written in a way where I talk about the attributes gained from those hobbies etc. and that way I can leverage those attributes to indirectly answer the 'why me' section that isn't explicitly included in the firm's criteria.
No idea if that made any sense lol...but it makes sense to me. So thank you!