In Win500_COM.exe, does the Serial Configuration window also show "(already open)" for COM4? If so, and if you select COM4, I would expect a warning after clicking the OK button.
I also have an acer netbook and I had the same error message. I went to greamerica.com and downloaded the drivers for 30-3290 csble. Works just fine now, win500 reads from scanner. Virtual control works fine. Acer sends to scanner no problem.