Smallest Camera

[HeroXLazer]

-No budget

-4K 24FPS

-Interchangeable Lenses

-Mount doesn't matter

-Really small

-Would like viewfinder but not needed

[HPWebcamAble]

How about a GoPro (Since no one else has suggested one)

[HeroXLazer]

I thought about gopro but no interchangeable elens

[ALwin]

BlackMagic Micro micro studio camera 4K?

https://www.blackmagicdesign.com/products/blackmagicmicrostudiocamera4k

 

With this camera, you will need to invest in a few accessories.  A viewfinder or external monitor for viewing what the camera sees and lenses.  It's also a "studio" camera so you need to invest in a external monitor+recorder combo, like the BM Video Assist or an Atomos recorder.

 

There also this camera: http://www.bhphotovideo.com/c/product/1203955-REG/z_camera_e1cam3a701_e1_mini_4k_video.html

 

and this from Canon which costs around $5000.

http://www.newsshooter.com/2016/04/13/canons-me200s-sh-cube-camera-a-c100-mkii-in-your-pocket/

 

Other cameras would be the Panasonic G7 or Sony A6300.  Here are other 4K mirrorless cameras:

http://www.bhphotovideo.com/c/search?atclk=Video+Resolution_4K&ci=16158&N=4288586281+3960940197

 

More 4K cameras (apart from the Panasonic DVX200, every other camera allows you to swap lenses):

 http://www.bhphotovideo.com/c/search?atclk=Resolution_4K&ci=28624&N=3705627354+4132992620

[EliminatingAngels]

Panasonic GX8, G7 or GH4 are probably the smallest 4k ready cameras with interchangeable lens system 

[xQubeZx]

Sony A6300. A lot of lens options since it has started to come up a lot of better adapters and sony releasing new lenses. For example the new Sigma smart adapter seems to be almost as good as using a native lens (so it says at least) for a very decent price of 250£. 

[ShadowCaptain]

A6300 for sure