SIO was written against the ST2 code base, but since ST2 and Ext.JS 4 have a common base, SIO will work with Ext JS 4. However, we have only tested SIO with ST2, and not with Ext JS 4, so although it might work OK, it also might not. So, OK for experimentation, but not for production. By all means try it, and let us know how it goes.
Currently it works with ST1 with ST2 support being added. There is no public dates for Ext JS integration although the ST2 classes may work... but may not.
SIO was written against the ST2 code base, but since ST2 and Ext.JS 4 have a common base, SIO will work with Ext JS 4. However, we have only tested SIO with ST2, and not with Ext JS 4, so although it might work OK, it also might not. So, OK for experimentation, but not for production. By all means try it, and let us know how it goes.