4K, 2160p and 8.3MP all basically refer to the same 'thing', just different dimensions of it.
While total pixel count has historically been the most common technique (e.g., 1MP, 2MP, 3MP, 5MP, etc.), naming by vertical count (e.g., 720p and 1080p) have become more common in the past few years.
Now, naming by horizontal count / lines (e.g., 2K, 4K, etc.) is a growing trend.
In this note, we contrast the pros and cons of each element and how to best use them in designing surveillance systems.