An i5/8GB would be preferable if you intend on using it as your main work machine, but I'd say that you could more than get by quite easily on 4GB, considering your requirements.
I ran a 4GB machine for years, upgraded from XP to Windows 8, and never ran into any memory issues. The only thing I'd suggest is keeping your multitasking relatively light. Jumping between a browser with 10+ tabs up and a big spreadsheet in Excel would be fine. Won't even notice a hiccup. But a browser with 10+ tabs, Lightroom and Photoshop with a heavily layered 20MP raw image on deck, and a big spreadsheet in Excel will be pushing it a bit.
As for your question, I'd say they'd be about the same in performance, with a slight advantage going towards the SP4, since it has the newer gen processor. They're both about the same size, both have the same thermal limitations, and are running roughly the same resolution screen. They'll be about on par with each other, not considering any of the various perks or detriments you get from their respective OSes.