update sdk ver

Rx.NET/Source/global.json

@@ -1,6 +1,6 @@
 {
   "projects": [ ".", "Tests.System.Reactive" ],
   "sdk": {
-      "version": "1.0.0-preview2-003131"
+      "version": "1.0.0-preview4-004233"
   }
 }