Would you trade an iphone 4 for a 7 inch HP tablet?

IPhone 4 <> HP Slate 7

No; it seems that iPhones are able to keep their market value relatively well because of their extreme popularity. Considering the tablet is only seven inches, it's not too impressive. Unless it's somewhat high-end or runs a recent version of Android, it's probably not worth it; the iPhone has a really good music player, swift interfaces, and is a phone after all.