Apple’s tech specs of the iPhone 14 reveals that 4K ProRes video recording remains unavailable for models that have less than 256GB storage.
For iPhone 14 Pro units with 128GB capacity, ProRes recording is limited to just 30 fps and at a 1080p resolution. To unlock the feature’s full capacity, Apple recommends an iPhone 14 Pro with either 256GB, 512GB or 1TB of storage space.

The lack of 4K recording is largely tied to the space in the iPhone- 4K videos in general tend to take up a hefty amount of data. The Cupertino-based company outlines how a 4K ProRes video takes up 6GB of space, while a 1080p video only takes up 1.7GB.
ProRes was introduced on the iPhone 13 Pro and refers to codecs that compress files into smaller sizes without compromising image quality. ProRes video recording is not supported on the iPhone 14 Plus and iPhone 14.