Are you tethering with Wi-Fi or Bluetooth?
Have you tried to speed test on a different Wi-Fi to see if both the n4 and tablet get the same speeds then?
That would indicate if the tablet is being slow or the n4 being slow on delivering the data. Also when you are speed testing make sure nothing else is running and using the connection in the background on the same time and that you have a password and only one device tested if you are using WiFi.