You wont guarantee that either will OC to 4ghz. But yes, the 1600 and the 1600x are the same chip and with the right cooling both should hit 4ghz. The only difference is the 1600x has XFR and it does not have a cooler. XFR will allow the chip to boost an extra 100mhz in certain circumstances. This is not apart of an overclock.
To me, the 1600x does not warrant the additional cost of the CPU and a cooler as cooler included on the 1600 is pretty good for a stock cooler. But, you probably wont get 4ghz with the stock cooler, you will probably get closer to 3.8. So if you want 4ghz you will need a new cooler and I would go ahead and get the 1600x.