The reason why 4K resolution is not followed by a 'p' like 1080p and 1440p is because the 'p' in 1080p and 1440p stands for progressive, which refers to the way the pixels are scanned on a display. 4K resolution, on the other hand, is a marketing term that refers to a display resolution of approximately 3840 x 2160 pixels, which is four times the resolution of 1080p.
The 'p' in 1080p and 1440p indicates that the resolution is progressive, meaning that the pixels are scanned in a progressive manner, rather than interlaced, which is the way pixels are scanned in a traditional TV signal. 4K resolution, on the other hand, is a resolution standard that is defined by its horizontal pixel count, rather than its scanning method.
In summary, the 'p' in 1080p and 1440p refers to the scanning method, while 4K refers to a specific pixel count. Therefore, 4K resolution does not need a 'p' to indicate that it is progressive, as it is already implied by the term 4K.