The most likely cause is the Arduino pin going to 4 volts, rather than 5. Have you measured it?
With that said, you are going about your gate drive in exactly the wrong way. You should not put any sort of divider before your gate. You need to understand the FET specs much more than you do. The upper limit on Vgs(th) is not a do-not-exceed limit. Rather, it is a minimum requirement. Worse, if you look closely at the data sheet, you will see that this level is specified to produce a drain current level of something like 250 uA or 1 mA, neither of which is remotely adequate for your load. For normal loads, you should provide at least 3 times the upper Vgs(th). In this case, if your load requires relatively little current you might get away with 5 volts. Then again, you might not.
EDIT - Since you have verified that the Arduino pin goes to 4.92 volts, the most likely culprit is your meter. 1 Megohm is the standard input impedance. Consider its effect on the circuit
simulate this circuit – Schematic created using CircuitLab
which is close enough to your 1.6. And if your resistors are not 1% tolerance, such tolerances could easily explain what difference remains.