I want to know why you have to buy both 4+5. If not how do I only download one? Same goes for buying 1+2+3.

Teeny tiny little games are bundled like that; another example being Alone in the Dark and first few WIng Commanders. I suppose if you already own one it could be a pain but you are still playing less here for the lot then you would anywhere else for just one of them.

As for installing, there is usually no way to avoid installing both but again the games take up so little HD space it really shouldn't be a problem.

With regards to installing: You'll need to download the whole package (4+5) initially, but during the installation process it does give you the option of choosing whether you want to install both, or only one of the games.