In your VBA, presumably, you are using usb_tc08_open_unit to open a single unit. This function returns a handle to that unit. If you want to open 4 units, call usb_tc08_open_unit four times, storing each of the handles. i.e.
handle1 = usb_tc08_open_unit()
handle2 = usb_tc08_open_unit()
handle3 = usb_tc08_open_unit()
handle4 = usb_tc08_open_unit()
If you need to find which handle corresponds to which unit, you can use usb_tc08_get_unit_info to get the serial number for a particular handle.
You can then use usb_tc08_get_unit_info or usb_tc08_get_single to read the temperatures.
For full descriptions of the API functions, see:http://www.picotech.com/document/pdf/usbtc08_en.pdf