Ellis - save the variables to the handles structure as
function pushbutton4_Callback(hObject, eventdata, handles)
filename = uigetfile('*.*');
myimage = imread(filename);
[BW, xi, yi] = roipoly(myimage);
handles.xMin = min(xi);
handles.xMax = max(xi);
handles.yMin = min(yi);
handles.yMax = max(yi);
guidata(hObject, handles);
In the above, we create new fields in the handles structure and then save that updated structure using guidata. Now, any other callback that fire (after this one) will have access to these four elements.