dahua nvr2104hs-p-i2 Camera Capacity

I have a client site with a dahua nvr2104hs-p-i2 (4 ch nvr) He currently has 3 x IP camera's attached. He has requested an additional 2 to be installed.
Can I add aditional camera's via seperate POE switch or are we limited to just 4 on this NVR?
Thanks in Advance.