I bought my SP4 not very long ago - maybe a month or two max. I had originally not planned to buy it as I was happy with my SP3. But I noticed that the screen was yellowing very fast from the edges and fearing a screen problem and the fact that I use the Surface as my primary device, I opted for the SP4.
But I have been scouring this and other forums on the SP4 and noticed with alarm (like the OP!) the various issues that seemed to be plaguing the SP4 (and the SB). Nevertheless, I went ahead and bought a SP4 (i5/8GB/256GB).
Out of the box, I noticed nothing out of the ordinary where the OS and stability was concerned. Of course the screen and finish are gorgeous. If I remember correctly, I applied the AU and a firmware update and a couple of cumulative updates. In short, my device is currently running fully updated. I am also in the Insiders' Release Preview ring. And, I am also running O365 (Insiders) on it.
I have to point out a few things.
Battery life: Granted I don't push my device. But I do move around a lot and I am constantly on the device. I get about 7-7.5 hours consistently. While I have set the screen brightness to low, I have left almost all other background services running. I would say I am satisfied with the performance thus far. Yes, a little more battery life would be great and I am sure MS will at some point optimize the OS further.
Stability: Now, contra the issues being reported here and elsewhere, I can't say I have encountered any of them thus far (touch wood!!). This does not mean that I am denying the problems that are being reported. I am just saying that in my experience with the machine I have not encountered them thus far. In my experience, the device has been stable and, thus far, reliable.
Heat: One of the issues with the SP3 was the heat generated even when doing mundane tasks. This is not happening on the SP4, which also means, I have not heard the fans come on except one time when that firmware was being installed. In my experience, the device runs cool.
Blessing: They lost that Windows button on the screen!!!
As for whether you should buy it or not?
If you need the device, yes, I think the OS and the hardware have settled down quite a bit. Compared to what I read about the state when the SP4 was first released, I think the state the OS and the SP4 is right now is what the initial release condition should have been.
Likely that MS will refresh the SP4 as a SP5 in the near future - be that towards the end of the year or at the beginning of the next. If the SP4 is considered to be an "evolutionary change" of the SP3, then the SP5 will be a further evolutionary change over the SP4 and I suspect that the degree of change between the SP5 and SP4 would be less than between the SP4 and SP3.
So, if you can wait till the SP5 is announced, then yes, perhaps you should as the difference that you will experience between the SP3 and SP5 will be noticeable. But if you need your machine now, then the SP4 will not disappoint and you would not lose out much by not waiting for the SP5.
On the 4GB RAM question: My SP3 had 4GB RAM and for the tasks that you mention, it worked just fine. But maybe it is me, I do notice however, that the SP4 seems to be a bit quicker on various things. But it could just be me imagining things. If your budget allows you, I would recommend getting the 8GB version as it will future-proof you to some extent, which means you could probably - all things remaining constant - wait it out without any real inconvenience for the SP5 or even the SP6.
Good luck!