Need helps on Dell Alienware Alienware Aurora R4 Video ports

Hi,

Below are the video ports on my Dell Alienware Aurora R4. I have used 2), 3) and 4) for 3 monitors. It supports 4 monitors. But I don't know what the 5) is.

Can anyone please tell me what kind of cable I should buy ?

1) Air Vent

2) DVI - I (Used)

3) DVI - D (Used)

4) HDMI (Used)

5) DON'T KNOW WHAT IT IS

Thank you.