I'm not sure my experience is necessarily a good guide for anyone else, but here goes: I had a SP3 for over a year before I got the SB. I used the SP3 mostly as a desktop, installing it in the matching dock and running a bigger monitor along with a wireless keyboard/mouse. For whatever reason I rarely used it as a tablet. I didn't get the Book as a replacement for the SP, but that's what it turned into. I now run it mostly with a dock and big external screen, but I have found that I take the clipboard off and use it by itself more than I ever used the SP3 as a standalone device. The clipboard screen is slightly larger but seems a lot bigger. I don't really use the Book as a carry-around laptop because I have a creaky old VAIO laptop with roughly the same size display that is usually adequate for routine purposes -- and if it should get lost or damaged while I'm out of the house with it I won't be as upset.
I didn't think the specs on the SP4 made it look improved enough to warrant moving up from an SP3. On the other hand, an upper-level book with the extra GPU in addition to the bigger display and the configuration options seem like a big and worthwhile advance. In my opinion the keyboard on the Book is much to be preferred to a type cover for an SP.
Then again, the SP4 with a type cover won't cost anywhere near as much as an upper-level SB, and sticking with the SP3 is even less expensive that that.
Hope this helps. I don't want to come across as an enabler, but remember that too much is always enough.