We have Surface Pro 3 i5's with 8Gb ram and it is more than sufficient for all desktop tasks. Pro 4 is likely to be better so you can probably get away with les ram, but just spend the extra money and avoid disappointment
I must say that I have a Surface Pro 3 with an i5 and 8GB of ram. It runs virtual machines, edits photos and videos using the Adobe Suite, and easily hammers out data in Excel or manages i can also access databases with ease. I do all of this and more on my Surface and not once has it slowed down or had an issue. I absolutely love it!