Ride quality between an equally equipped Lariat & FX4 should be identical. There is nothing in either trim level that would change the springs for that truck. As stated earlier, the springs are computer selected based on the trucks configuration. Those "default" springs can be modified by options that require stronger springs.

The only other variable factor in the ride quality equation would be tires & rims, but all OEM combinations offer at least 5.5 - 6" of sidewall so none of them should really change the ride quality that drastically.

Regardless, the point here is that any two similarly equipped F-150s (regardless of trim level) will share the same ride quality.