Always best to say where you are from as this is an international site. Monetary changes drastically depending on country.
See you are in L.A., USA so talking $$USD
You are not including cameras in this question? Cams are big ticket items.
Was the reference to 4K for monitors?
Yes you can buy a nice set-up for $15,000 USD, but it totally depends on what type of work you'll be doing. Some shops have way more than this invested for set-ups.
A very decent MacPro w/o Apple ram upgrade will set you back around $6,000 so if you go this route this will eat your budget totally especially when you include a $3,000 @4K monitors.
The iMacs with the i7, 3rd party ram upgrade, best GPU will set you back around $3,500 and includes a very nice 5K monitor. This is actually a nice machine to go with, but make sure it is the 27" 5K retina model.
You will need external drives, both for media and back-ups. This is especially true when working collaboratively and will add anywhere from a $1,000 to well over $10,000 (really skies the limit) Let's say 3-5 grand.
Sound can be either basic, decent sets of active monitors and cans, or very elaborate with mixers, mics, amps, etc and could therefore be any $$$$
That leaves software.
FCPX, Motion, Compressor to get started is a true bargain @ $400
Budget for plug-ins, and whatever other software you think you need.
Don't forget the expresso machine and fridge for beer.
Hope this helps, Greg
The Mac Pro supports up to 3 4K monitors via Thunderbolt and HDMI.
Be aware the HDMI port shares a bus with TB #3 port.
Up to 60Hz if I remember correctly.
You can also hang a 4K display off the iMac, but I totally forget the details OTOMH.
Apple has to supply the driver, so check compatibility for any monitor you are interested in.