For Ibanez's line of guitars that utilize the Gibraltar bridge, the Hipshot Ibby HM is a direct retrofit.
Although the Ibby HM uses only two screws, as opposed to the four on the original instrument, the body of the Hipshot bridge covers the two remaining holes. Two screws are more than sufficient to hold the bridge down because the strings are mounted through the body and over the bridge, clamping it against the body.
For Ibanez guitars that use the flat plate style bridge. The Ibby FP will be a direct retrofit.