I believe this came about because airlines asked for it. At least that is what I was told happened at STN were the same 170 to 5 has been in use for some time now.
160 to 4 doesn't work on on our type because the F5/F10 speed is often in the 165K range. We don't lower the gear until 4nm on a standard low drag approach (weather permitting), so would have to lower the gear early (and waste fuel) to select F15 to comply with 160kts.