Any other Web browser plug-ins and or add-ons aside from the Flash Player? Try disabling all plug-ins and add-on except the Flash Player.
Does YouTube video, 1080p videos and higher, streaming works fine?
As for the Roku4, if you decide to get it, try waiting for a sale. If this is your first Roku, then you may get Roku4 or Roku3. Performance is pretty much the same with Roku3; major selling point of Roku4 is the 4K.