Greetings,
Your board has 4 ports. You say you have access to Ports(0:3), which is port 0, port 1, port 2, and port 3. That is four. If you need access to the fourth port then you will reference Port(3) in VB. The issue is that VB indexes the array beginning with 0 and not 1.
Regards,
Justin Britten
Applications Engineer
National Instruments