I don't see an issue generating a single 4k DCP from mixed 2k and 4k sources. I
don't think any degradation will occur.
Do you need to create them all in one single DCP? If there is no specific reason to do
this, why not make every one at it's 'native' resolution, and the cinema will
have more freedom to create a playlist?
In general - the only reason not to create 4k DCPs is if there is a chance that some
smaller cinemas would play them with inferior software players. Some of them can not or
will not play 4k DCPs, or need an additional license for 4k. All proper DCI servers will
play 4k, even if the projector only supports 2k.
- Carsten