I'd say not enough. Peach is a very mild fruit in flavor and very low in acidity so once you ferment out the sugars there isn't a lot left to give it a distinct flavor in the beer. It's my experience that it turns in to sort of a mute, generic fruit flavor. I've had other peach homebrew and generally found that to be the case although some people really seem to nail it. Not sure if there was some peach extract in play to get a good flavor. I dunno, I'd probably blend peach and apricot because apricot will add some acidity myself. Regardless of what you choose to do I think 4.5 pounds is too little.