How does 4K compare to 35mm film?
Most 35 mm film outs that have gone through a scanning process have been scanned at 2K, or one quarter the resolution of 4K. Only extremely high content VFX work is generally scanned as high as 4K. Viewers that see RED footage for the first time either describe the quality as 65 mm film or “grainless 35″.

What are the advantages of shooting digital vs. film?
Beyond the obvious advantage of cost savings with the elimination of film and processing, both consumable products, it is the benefit of being able to watch dailies immediately, in real time, that make shooting digital such a superior experience. In adding the extraordinarily high quality of 4K capture and at such an affordable price, RED has tipped the scales for the industry.

Technology
Digital cinematography captures motion pictures digitally, in a process analogous to digital photography. While there is no clear technical distinction that separates the images captured in digital cinematography from video, the term “digital cinematography” is usually applied only in cases where digital acquisition is substituted for film acquisition, such as when shooting a feature film. The term is not generally applied when digital acquisition is substituted for analog video acquisition, as with live broadcast television programs.
High-end cameras designed specifically for the digital cinematography market often use a single sensor (much like digital photo cameras), with dimensions similar in size to a 35mm film frame or even (as with the Vision 65) a 65mm film frame. An image can be projected onto a single large sensor exactly the same way it can be projected onto a film frame, so cameras with this design can be made with PL, PV and similar mounts, in order to use the wide range of existing high-end cinematography lenses available. Their large sensors also let these cameras achieve the same shallow depth of field as 35 or 65mm motion picture film cameras, which is important because many cinematographers consider selective focus an essential visual tool.