4K is a very wide term and depending on what format that 4K is in it can be cheap or it can be very expensive.
Is the 4K:
-Uncompressed? TIFF, DPX, EXR. You’ll need insanely fast drives to work with these.
-RAW? R3D, Arri, CinemaDNG, etc. You’ll need a combination of fast CPU and GPU to work with these. Depending on the compression you’ll probably need SSDs for media drives.
-Nicely compressed? Prores, DNxHD, other “visually lossless” pro formats. These work pretty nicely with basic hardware up until you want to put effects on them.
-Heavily compressed? H264, H265 usually from DSLRs or cheapo cameras. These are usually really, really intensive to decode even on faster hardware and I would suggest to transcode these to better codecs like DNxHD or Prores.
Also do you need to run these in full res or can you work in half/quarter/eighth quality? Do you want to use proxies? This can bring the requirements down a bit.