After Lumia 822, 928 and 1020, Nokia has now submitted application to FCC for upgrading Lumia 925 to Bluetooth 4.0. The application seeks permission for upgrading both Global RM-892 and AT&T / T-Mobile variant RM-893 to Bluetooth 4.0 with a software update.
There is no such application for Lumia 920 as of now, but Bluetooth upgrade looks quite possible for it now. This also indicates that GDR3 / Nokia Black may bring this upgrade for most of the Lumias out there. We will keep you updated :).