They will 'work' but not well. That NVR only has 40Mbps incoming bandwidth. The NVR can handle up to 4 channels or up to 40Mbps, whichever is reached first so you would be limited to the settings you could apply on the cameras in order to keep the total combined bit rate from exceeding that 40Mbps limit. The 8 channel version would give you 80Mbps to utilise so would be better (but just for 4 cameras).
You would be better investing in a higher end NVR if you're intending to use 8MP cameras. The best option would be the 7608NI-M2/8P or down from that the 7608NXI-I2/8P. Those NVR's have higher incoming bandwidth to support high resolution cameras with a higher bit rate requirement. Additionally they have better decoding capabilities for live viewing and playback.