Two comments:
1) If you have a single yaw bearing it should be about as high as the center of effort of the turbine's drag, i.e. near the turbine axis.
This is especially true when the bearing is narrow and a press fit on the mounting pipe: The drag of the wind is applied via a lever arm corresponding to the distance between the center of effort of the wind from the bearing. As the wind varies the shaft wiggles back-and-forth in the bearing - eventually working its way out.
With two bearings placement is not so critical. The leverage turns into side thrusts in opposite directions (both below or both above the center of effort) or in the same direction but at possibly different strengths (one below and one above). The "tapered yaw bearing above the turbine axis, pipe-over-pipe below it" approach is such a two-bearing situation.
2) The narrower a pipe (or other beam), the weaker it is. Your two-inch pipe inside the four-inch pipe is much weaker in bending than the outer pipe and prone to falilure. And it did fail - at the maximum bending-stress point (at the upper ring). You should have used two large pipes, not one big and one small. Or done the reduction immediately below the yaw bearing (with the latter about the height of the turbine axis).
