To me Type Cover looks like good investment- I like bigger touchpad and spaces between keys, as more often than not, I'm pressing two at a time. Also, I will have that bit less to buy with new SP4.
Pen is tricky part: according to this blog-
Surface Pro Artist SP4 Pen will not have 1024 levels of sensitivity on SP3, although it is still great improvement in line smoothness and general feel. If I planned on keeping SP3 longer (perhaps even until SP5 is announced, as I don't have any problems with my unit and I'm happy with it's performance), then I'd buy SP4 Pen now. Second scenario is me buying SP4 early next year- additional Pen isn't necessary then, I'm quite ok with older Pen model