Online documentation for the Honda X4 appears to be somewhat scarce, but the engine was also used with little or no modification in the Honda CB1300, although that configuration used slightly different gear ratios that produced less low-end torque but lower engine RPMs at cruising speed. Specifications for the CB1300, including gear ratios for the configuration of the engine and transmission as used in that bike, can be found here:
http://www.hondacb1300.net/cb1300specs.htm
Other than swapping CB1300 transmission gears for your X4 set, I am not aware of the general availability of custom transmission gears for your bike's transmission, and I suspect that they might have to be custom fabricated by a precision machine shop.
In contrast, a simpler and much cheaper solution to reducing your highway speed engine RPM would be to tinker with the forward and rear sprocket sizes for your chain drive. Adjusting one or both of these sprockets by even a single tooth (larger in the front, smaller in the back) can have a noticeable effect on the cruising RPMs. Sprockets, being normal wear and tear items, are available in a variety of sizes in the aftermarket, and you should be able to find a variety of sprockets that fit your bike. I would suggest that you start with this, as this modification is both vastly cheaper than rebuilding your transmission and easily reversible if you're dissatisfied with the results.