i think the PRO has something to do with performance, it's better that you have a MSPRO.
but then having a MSPRO 4gb in a k750i is too much...
oh... and one thing... the phone's record sound function doesn't work whenever a 4gb Memstick is installed. that's the major problem with the 4gb.
i recommend using a 2gb SONY MSproduo, the black one with the 80mbps, it's faster...
i am using it, and it's great