4MP cameras are an important emerging trend in video surveillance, aiming to replace 1080p.
This is being driven by new sensors from OmniVision that deliver almost twice the pixel count of 1080p at the same 16:9 aspect ratio. These sensors also include true WDR. They are completely unrelated to the 4MP effort from ACTi from a few years ago.
The diagram below shows the difference in pixel count:
The two companies most aggressively pushing these 4MP cameras are the Chinese mega-manufacturers, Dahua and Hikvision. Not surprisingly, they are offering these cameras at extremely low prices - 4MP, integrated IR, true WDR fixed focal domes for under $300.