Yes, it's a PSP game. However, it would be a mistake to discount it and assume it can't possibly live up to the expectations of a game like MGS4.
While it certainly can't boast the technical prowess of a PlayStation 3 title, Hideo Kojima promises that Metal Gear Solid: Peace Walker will be a full MGS experience. According to Joystiq , Kojima says "the main staff of MGS4 are working on it," and the master has already said Peace Walker is like "the MGS5 he has inside of him." The team's goal with this PSP production is to aim for PS3 quality, and they will attain this goal by really pushing Sony's portable hardware; "expressing this in ways only we can express it in." And if you're wondering about the base gameplay, Kojima says it'll be "stealth-action based" like other entries in the franchise, and we can also expect some customization for Snake and a "growth feature" that allows your character to "evolve." This latter feature is one we're definitely going to want more information on, primarily because it should serve to enhance the depth. Many are concerned that the limitations of the PSP hardware would hinder Kojima's ability to create something deep and technical, but the day we doubt this guy is the day we stop playing games altogether.
In fact, considering the fact that Metal Gear Solid: Rising should be more action-oriented, the hardcore aficionados might be more interested in Peace Walker . It seems as if it will follow in the well-established tradition much more closely.
Related Game(s): Metal Gear Solid: Peace Walker