iP4 is a phone ( 3.5" screen ) while the SGT is a tablet ( 7" screen ) first, phone second. There is no rationale behind this change unless you already have an extra phone on hand or you find that the iP4 is of no further use/or you are bored with it.
Both are also completely different O/S. Having use the iPhone 4 before and present iPad - the OS of each caters to a different market of users. If you ask me based on my own needs I'd say the SGT is a better tablet to suit my usage compared to the iPad.

However I feel the phone function in the SGT is only a bonus . Making a call is at your control - you need not worry about how , where and when you want to do it. But recieving a call on the SGT will pose a problem depending on where and what situation you are in. Unlike a phone, you just cannot whip it out and just take the call. The SGT poses more inconvenience especially while you are on the move ( whether driving or just walking ) - just imagine the trouble you'd take just to take the call