I think this is probably an area where you could get away with either device and you have to consider the greater of two evils. I.e. is battery life more important than some other aspect you get with the SP4. You've also specified note taking but I think you need to consider the outcome of the notes. I.e. what do you do with them and where will they be stored.
From a stylus perspective, the latency is definitely better on the iPad Pro over the SP4 but for handwriting both will work so it's not that one is unusuable. I find the Pencil feels more natural but that's subjective. Ignore the issues of it breaking off when plugged in, you can still charge from the cable and the plug in issue is only really for situations where your battery runs flat in a meeting or on a plane.
Things I would also consider is what application you intend using it for. For example, if it's OneNote, the desktop version of OneNote on the SP4 is vastly superior to the iPad Pro version. The IPP version may get better in time, but right now it isn't which is more of a MS issue with how they have developed it than an Apple issue.
Things missing from it are:
- No conversion to text option. If you use Office 365, you can still search the handwriting on the IPP, but you don't have the capability to convert from handwriting to text if you want to convert handwriting to text for distributing minutes of a meeting.
- No templates. OneNote for Desktop allows you to create templates for minutes of meetings, workshops etc
- There is no option to scratch back and forth to delete something (as if you would to scratch something out on a page) which I find is fairly critical for handwriting as it helps speed things up.
- Handwriting has lag on the OneNote IPP version vs other apps (more of a MS issue than Apple issue because other non-Apple apps don't have this issue).
If OneNote is not part of your core workflow, then there are other IPP apps (like Notes Plus) that will do what you want, which is why I said you need to consider not just what you are using it for, but what apps you intend using it with. If you are using OneNote regularly, then you need to consider which device is better and for me, the SP4 is better with OneNote or requires work arounds. For example, if you wanted to convert notes to minutes, you would probably have to use something like the Notes Plus app, convert to test and paste it into OneNote which may not be a palatable option.
There are other small things to consider. For example, if you travel a lot on planes that don't have power adapters, the iPad Pro may suit better because planes still have USB ports which do charge although I haven't tested with an IPP to know whether the charge is sufficient for the size device. It does work fine on an iPhone.
If battery is key, and functionality secondary, I would go iPad Pro.
If OneNote functionality is key and battery life is secondary, I would go SP4
If both are critical, I would get a trial of both devices and give back the device that doesn't work as well for you. You may find that things you thought were important aren't and things you thought were important aren't.
As I said, it's the greater of two evils to consider.