Does NOT fit the TRX-4 : it is 20mm center to center, should be 22mm.
The end of the longer original horn fit into indentations in the servo mounting plate.
Since this SSD horn is shorter, the end ball does NOT fit in these indentations, and the servo travel needs to be significantly reduced.
The reduced length and reduced servo travel mean a bigger turning radius, not good.
I'm dissapointed and cannot recommend this.
(My TRX-4 is 100% stock.)