The SP4 will frustrate you to no end. I say this as someone who's house has two android phones, to iPhones, three iPads, a linux server, and 6 Windows machines (laptops, tablets, and desktops). Here's why:
The current crop of apps available for Windows tablet mode are very limited in functionality, poorly maintained (in general), and there is very little choice. Some apps which are stock/standard on both android and iOS don't exist in *any* fashion - paid or free. Many seem like they have MS equivalents, but you will find that those are Windows Phone only. Continuum is coming, but I would anticipate any crossover to be a minimum of a year off. You can emulate Android (DuOS), but I wouldn't do that without an 8GB machine (which I have) and it still hammers the CPU and drains the battery. And the apps are not nearly as friendly as the iOS ones. Anyone with ForScore will laugh at you while you struggle with Reader or any other MS/W10 app for reading music.
The battery life will kill you. Not literally, but it will make you very sad. Battery life is 4 hours. If you must guarantee longer than that (like, you are a hour from the office and billing $200/hr - or are on stage for a gig and need it to Just Work [TM[) you need to have an external battery or a charger. Period. Yes, I've gotten 6.5 hours on a charge. I would not put $600 in billable time, or a live gig, on the line, because I've gotten as little as 3.5 hours of useful time on a charge without doing anything intentionally power-hungry.
The standby time is also awful. And when I say awful, I mean when compared to a MBP, and iPad, an Android tablet, or even some cheap 10 tablets (like the Best Buy UnBranded one I have). This is AFTER the recent update that "fixed battery drain. My original SP4, upgraded to the current "battery saving" drivers, burns 7%/ hour when sleeping (8.5% if a USB device is plugged in) and 1%/hour when OFF or in hibernate. It means that after I finished my rehearsal last night with 50% battery and I let it sleep, then doze to hibernate, I woke it this morning and it only had 20% battery left. Now...to be fair, MS is replacing this device.
I have a brand spanking new SP4 (a replacement) I'm setting up, and - from scratch - it has about a 1.3%/hour drain in sleep mode, and about 25-45mW drain when hibernated, but I haven't actually installed any software on it yet. I'm hoping it doesn't cause more phantom drain. This still means that it will lose 20% of it's charge per week while just sitting on a table.
Okay - that's all the sucky stuff. Are you still with me? Are you still so delusional that you're thinking, "I'd give up all that if I had a 2lb tablet that had a real pen and could be my only computer. I can tweak a bit, and I'm willing to spend some time customizing to get it jsut the way I want it." Really? Then run, do not walk, to the MS store and get one of these babies. Because - damned - they are about as amazing as it gets. It's just light enough to be a tablet - not Air light, but comfortable for all but the weakest wristed, Starbucks soy latte drinking metrosexual (not that there's anything wrong with that). And with the typecover, it's a real everyday machine.
I have an i5/8GB/512GB. If you aren't going to store much on it, the 256 is probably fine. In fact, if you're really tight on money, and you won't run a virtual machine (or second OS like DuOS), you can probably live with the m3/4GB/128 and get a 128GB uSD for $50 to put your OneDrive or Dropbox folder on. The m3 is, surprisingly, not much slower in practice than the i5. The i5 will sustain high speeds for longer, but the m3 is within 5-7% of the speed for short bursts. Here's the breakdown:
m3: general use, need to stay quiet always (no fan)
i5: general use with some heavy computations - Photoshop, Premiere, DAW work
i7: you really want to game on the run
note: if you do so much video that you think you need the i7, you don't want the SP4, you want a real desktop, or at least the top of the line, 4 core rMBP.
4GB: general use, no virtual machines (that includes Android emulators)
8GB: Video editing, one virtual machine, reasonable sides CAD work (but not huge)
16GB: you're a developer, or you're working with really big video, or some other speciaized program that *requires* more than 8GB to run.
128GB: you don't store anything locally
256GB and up - whatever you think you have to have with you and are willing to pay a 300% premium (per GB) over a laptop drive cost to keep.
I run everything on mine. I run two 4k monitors with AutoCAD. And I'm working on a 24+/- track vocal recording in Reaper. And doing some Premiere work for our video (simple, though - nothing really complex). Oh - and don't listen to the Chrome naysayers. I transcribed a bunch of handwritten (in OneNote) notes into a Chrome Sheets spreadsheet two nights ago - and was on track to get 6.5 hours of battery life. The only thing to avoid in Chrome is video. Doesn't matter if you use Flash/VP8/h264ify - Chrome really does hammer the CPU for video; use Edge for that.
Okay..that turned out longer than I expected. Good luck!