Add solution for Expert 4

Does this qualify as "at once" ?

I had wanted to use `@devec` to do this without the temporary, but for the moment (as far as I can make out) this appears not to be possible https://github.com/lindahua/Devectorize.jl/issues/31.

Also, fix question 3 (atm it's a repeat of 2).
Hmmm now I look at the numpy version this uses a 3d array rather than an array of arrays. So not the same atm, will have another look at this (unfortunately I don't think the solution will be so so elegant!)