Open and split a 4 channel RGB picture

How can I split an 4 channel rgb pic in qgis? With the value tool I can see 4 channel. I tried to use the RGB to PCT tool but I always get an error (File "C:\PROGRA~1\QGISBR~1\bin\pct2rgb.py", line 125, in ct_size = ct.GetCount() AttributeError: 'NoneType' object has no attribute 'GetCount' )