A really short explanation: Mint4win is the Mint-customized version of Wubi (Ubuntu). They allow you to install Ubuntu/Mint within Windows, as an added convenience for new Linux users wanting to make the switch but unable to comprehend the concept of partitioning their system. However, I personally don't recommend it as anything other than a stepping stone, reason being that Linux becomes nothing more than a program within Windows and if Windows goes down, so does your installation of Ubuntu/Mint...there are also other factors to consider, like the fact that installing Linux in a native Linux filesystem (ext2/3/4, reiserFS, brtfs, etc.) is more preferable than installing it in a Windows' NFTS partition, for example.
A Google search would've answered your question much more quickly though.