4K ProRes video recording makes some demands on the iPhone’s hardware. Not only must the processor and memory be fast enough, there must also be enough space.
Apple therefore limits the ProRes recording function on iPhone Pros with only 128 GB to 1080p instead of 4K. At 4K, such amounts of data accumulate that the memory would be full within minutes. Unfortunately, this is only briefly mentioned in the fine print. Probably many will not use this feature anyway, but for all those who are interested in maybe recording a few minutes in the best quality, this is an important hint.
Of course, it would have been nice if Apple had equipped all Pro models with 256 GB of storage, but that is probably wishful thinking. In addition, users who want the 4K feature will likely go for models with a larger storage capacity. It is not for nothing that Apple offers 1 TB models.