I apologize for any confusion caused by my previous response. I appreciate your clarification. To further clarify, while the resolution for 4K UHD in consumer displays is indeed 3840 pixels × 2160 pixels, which is not exactly double the pixel count of Full HD (1920 pixels × 1080 pixels), it is still often referred to as "4K" in consumer terminology for simplicity and marketing purposes.
In terms of pixel count, 3840 pixels × 2160 pixels does represent a substantial increase over Full HD, providing four times the number of pixels (2 times in each dimension, hence "4K"). However, the exact doubling of pixels is not maintained due to historical and industry-specific considerations.
I appreciate your diligence in seeking accurate information, and I hope this clarification addresses any confusion.