W3schools offers a better explanation on the ready-state-change here.
To answer your question in particular, it cannot be determined. While transitions between states are defined certain states may trigger the on-ready-state function multiple times (e.g. 3: processing request ).
Note - W3schools mentions that "onreadystatechange event is triggered four times, one time for each change in readyState.", but I remember of some place that onreadystatechange could trigger of multiple times, for the same state.