Actually no version of Bluez < 5 contains a working LE API. Bluez 4 has "something" (see doc/attribute-api.txt) but it lacks support for very simple GATT features such as subscribing to an attribute for notifications/indications.
Even the Bluez version on the N9 contains some support for BLE, and you can test it just by running:
on a root terminal.

Because they too use Bluez 4, the situation on the Jolla is exactly the same as on the N9: the hardware supports it, but there is no API to use it.

Bluez 5 requires kernel 3.5 (or at least the addr_type field in L2 sockaddr struct), which the Jolla doesn't have.
Many people with Bluez 4 use a tool called "gatttool" as GATT stack, which, despite coming within the Bluez tarball, actually mostly bypasses user-space Bluez and opens kernel bluetooth sockets directly. Additionally, because it is a tool, and not an API, you have to execute it, pass command line arguments, parse the result, etc. etc. This seems to be the approach Qt 5.3 is taking, which I'd not trust very much. But if Qt 5.3 is choosing this I suppose it means we're stuck with Bluez 4 for a while.