A 4terabyte Memory Stick Pro Duo would be so awesome that it's possible the PSP would somehow fuse with my body in some sort of absurd steampunk symbiosis.
That's likely an import cost (or the cost of doing business with Sonystyle the moment a product is announced). Sandisk is setting retail of 375 last I looked, and preorders right now for even the Sony's are set at around 350-370.
I'm not so hopeful about the price dropping below that this year because of pent-up demand and the fact that 1 and 2GB sticks won't start becoming that common until September.