Does 4K impact quality when filming in low light? (other parameters being equal)
There are three key factors in your cameras low light performace:
1. Lens (Aperture)
The lower your f-stop, the more light gets through to the sensor, you will also have a very shallow (narrow) Depth of Field. Often this is a requirement at night, since cutting off most of your light is generally a bad idea. When looking at any camera, see what lenses are available to suit your needs.
2. &quot;Pixel&quot; Size
Each little diode on your sensor collects light, the size of each diode directly effects low-light performance. There are two significant factors in the size of diode: sensor size and resolution. If you have 1920 pixels across your sensor and your sensor is an inch wide each diode will be about 1/1920 of an inch across if you sensor is 3840 pixels across your diode will be about 1/3840 of an inch across, much much smaller, and therefor able to collect less light.
3. Sensitivity
The sensitivity of a sensor can be controlled with ISO. The max ISO is very important for low light capture. Some excellent cameras in other regards have very low max ISO's, and are therefor unable to shoot in low light. Some cameras have very high max ISO's, and are designed for low light. Such cameras are the Canon C-Hundreds series, and the spectacular Sony A7s. The A7s has a relatively low resolution sensor and very high resolution.
The simple answer is yes, 4K will negatively effect low light performance, but most cameras already have a 4k-sized sensor that they simply fail to utilize (the canon 6D suffers equally from small diode size as the A7s for example). In your case, no 4K will have no effect, but if you're going to get a 4K camera for low light, buy the A7s and an external recorder. Out of it you will get 4K raw. For about $3k. It will put your 6D's performance to shame. Some people have lit A7s shots with moonlight/an iphone. Gulp.. did you see that last one, it out did the C300..... :O!
PS:
I first mentioned lenses for a reason. The sony A7s takes Sony E mount lenses. In my opinion they are some of the worst selection of lenses on the market. (Some are good glass, but there are no versatile zooms). Be wary of the lenses, they are half the key to getting your night shot. Also you HD will look amazing with a good lens, they've shot Hollywood features in HD (like Scorsese's Hugo), utilizing the power of $30k lenses.