I've never used a SP2 or SP3. I'm coming from a SP1 w/i5 4GB 128GB.
What will I notice the most moving to a SP4 w/i5 8GB 256GB? I use PhotoShop, Canon's Digital Photo Professional, Word, Excel, Modern IE (tend to have a lot of tabs open, 20-40) and am often multi-tasking w/ split screen. I would like to use it outside more but it is unusable in direct sunlight. I rarely attach the keyboard.
Will the SP4 be much of an upgrade?
You get improved performance. Boot time, program startups should be similar (because the SP1 has a fast SSD already), but doing CPU task it will be better. GPU performance will be much improved. The device is lighter and have much better battery life. It is a bit bigger but you gain so much height, it will help productivity (16:9 aspect ratio at 1080p is too cramp).
The pen tracking will be much much improved, and corners will have great tracking now, and the different tip options (probably sold separately) for different feel as you write/draw is interesting. It must be noted that, however, you'll be passing from Wacom to N-Trig. N-Trig has it's share of downsides. Now I am no artists, so you have to do your own research. It does have WinTab drivers (at least the Surface Pro 3) if you need them.
You'll enjoy a much improve keyboard, backlit, thicker and more solid. As well as a much larger and better trackpad with glass surface.
You'll have an improved kickstand which variable and larger positions allowing you to be more comfortable when using your system.
The system should run cooler and quieter due to more power efficient processor, and better cooling solution (but that is guessing now, as we have to wait for reviews).
More RAM will help you a lot in your work, so you'll have a nice performance boost on that front.
You'll also enjoy a more refined product. Microsoft has learned a lot since making the surface system, and have been applying great improvement in how to design things internally, to manufacturing at every version of the Surface Pro line up. The SP4, I expect to be very polished experience.
You'll still need to be careful with the device, it still uses a magnesium body, and that means that the body is painted and that means, like your SP1, anything will scratch it. The gray/silver color hides the minor scratches, but you can see the scratches.
You'll enjoy the much improved power connector introduced in the SP3, and a new dock option.
Finally, you'll enjoy a better camera on the device, if you care about them. They also feature autofocus (you are passing from 1.2MP to 8MP camera)
The device should provide you better sound as well. with what it looks like front facing speakers like the SP3, or internal speakers but pointing at you (hard to tell with the current images).
As for the Surface Book. The Nvidia GPU (option to take) will be a big boost in software that uses graphic acceleration. You have the same benefits mentioned above. In tablet mode (that is when you pull the screen out of the base), you lose the dedicated GPU (as it is on the base), and ports beside the audio headphone jack, and your battery life is reduced down to 3h from 12h (from Microsoft). But it is even lighter and thinner.
The screen can be flipped and put as a tablet.
There are 2 unanswered questions with the Surface Book:
-> Can it survive high pressure due to the screen not closing flat with the device?
->Will the edge back and screen (depending on which direction you flip the screen) get scratch, as there is no pads visible.
The above points shows potential problems with the system, which if true to be an issue, shows the lack of experience in making computer from Microsoft. I am sure the Surface Book 2 will correct the (maybe improve the hinge so that it can close completely, and have a rubber protection pad all around the screen like the MacBook Pro.
