My desktop is a little overkill for 4k as it was designed as a gaming machine, so it was a little pricey because of the graphics card which doesn't do much for Adobe Premiere Pro anyways. For a full build, looking at between $1500-2000 depending on the graphics card you choose and the amount of hard drive space. This is what I am running:
Intel Skylake i7-6700k 4.0 GHz, 32GB DDR4 2400, nVidia 980 Strix 4GB, 3x1TB SSD
My laptop is a couple years old, but still can process 4k without too much problem. The render time is a few times as slow as the desktop, but that is to be expected from an older notebook that was also meant for gaming. Editing the videos does not push the system too hard and there are very few times where the computer has to pause to catch up.
Alienware m14x, Intel i7-3630QM 2.4 GHz, 16GB DDR3 1600, nVidia 650m 1GB, 2x250GB (m.2 & mSATA) SSD