The GTX 1080 is a very powerful GPU, and would suffice for editing 1080p60 and 4K, although you don't say what format your footage is in. Being new to Premiere can be hard, so there's a lot to know, but editing 4K H.264 footage is not a great idea, and regardless of how powerful your machine is, you should always consider the most efficient way to work. You might be able to play back a 4K stream of H.264 with no problem, but as soon as you start stacking effects (especially if you throw any that aren't GPU-accelerated, you might have problems).
I have a GTX 1060 with a Core i7 7700K, which can be overclocked. It runs Premiere and After Effects great.
You should know that After Effects doesn't use the GPU in the same way that Premiere does, and it is not built for realtime rendering. It doesn't matter if you have 4 Titans in your computer, you won't get realtime playback on a heavy comp in After Effects. Being smart and working at half res with footage encoded in an intermedia formate, such as ProRes, Cineform, or DNxHD, is a smart way to work.
You could consider saving some money and dropping to a GTX 1070, and if you feel that performance isn't good enough you could always pop a second 1070 in, which is going to be more powerful than a single 1080. You can't run SLI on anything below a 1070, so a 1060 is out as an option there.
You mention that you want fast rendering and fast exporting, but again, that's not entirely hardware dependent. If you have a heavy comp with a lot of effects it's likely going to be faster to export to a master file in an intermediate format (again, ProRes, Cineform, or DNxHD), and then convert that flattened file to H.264. The same goes for rendering previews in your timeline. The preview codec is usually set up as MPEG I-Frame, which is a lower quality setting that you would not want to export with, but instead quickly generate previews. Going back to exporting though, you have to understand what's going on in order to optimize your speed.
You're decoding your footage, stacking effects on it, possibly blending those effects together, and then encoding as a different format. That takes a lot of processing power and it's a lot easier to encode that to an intermediate format that's easy to work with than to go directly to another highly compressed format like H.264. Taking an intermediate file which is flattened and doesn't require any processing of effects will convert to H.264 much, much quicker.
Another really important thing that you don't mention, which will drastically impact your performance is your drive type and speed.
If you're trying to read 4K video off a slow, spinning HDD, it doesn't matter how fast your processor is if your HDD is too slow to serve up frames. You need an SSD. You should also consider another SSD for your media cache and After Effects disk cache.
Also, regarding Premiere and After Effects, Dynamic Link is cool and it got a huge performance increase last year, but it's still slow and having Pr and Ae open at the same time shares resources. I'm working on a project right now where I'm going back and forth, but if I don't need Premiere while I'm animating, I close it.
There's a lot more to consider than just your CPU and GPU.
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