Even with the recommended amount of RAM, working with 4K 12-bit film puts great demands on memory management. Set the RAM allocated for buffering frames to between 2000 and 4000.
The ability to mix different resolutions in a 4K 12-bit film project increases memory requirements even further. Even with an override of the default memory management configuration, working at this resolution may not support some advanced features that require a large amount of memory such as the Colour Warper™.