The store property is a documented property of Ext.data.Model and as such can be relied upon for customer use. It was not present in the 4.1 alpha release and was reported at that time. It is also not present in the 4.1 beta release. It seams to be replaced with a stores property which appears to be an array containing 1 element.
I have not seen mention of this in any 4.1 release documentation or backward compatability guide. If I missed it my apology and please point me to it.
In the mean time can I rely on stores being an array with the first element pointing back to the store?