I'll take your incorrect abbreviation of Photoshop as proof that you've never used it before. Files with the extension .PSD are the format that PS saves to. PS is the abbreviation of Photoshop.
Anywho, I don't know how you can say it's been the same since v4, because there's been literally hundreds of new features added since then. Plus, PS is the original professional desktop graphics package, despite what anyone else may tell you.
