there are tutorials around this very site that tell you what you need to do to run a MS pro, basically the only differance is the form factor. its not very pretty because it hangs out of the psp but its not horrably ugly either, very possably worth the uglyness to have a 4GB stick.
just look for the tutorial in the pinned topics under this psp forum and you should find it.
i know where it is but its good to learn to look for things like this on your own.
i saw those 4GB sticks on ebay for around $300-350 about the same price as the 2GB duo sticks.