OK, how to say this. The Nexus 4 is a great phone.
As you've noted, post 4.3, you've got the HW/SW to talk to a Bluetooth 4.0 LE device, aka Bluetooth Smart, device. Awesome.
And you've decided that you will only use ONE app, Endomondo.
And you aren't happy that that app on that platform only supports a heart rate monitor, you want it to support a speed/cadence sensor too.
Terrific.
Then you'll want to ask in an Endomondo support forum which speed/cadence device they might support on Android someday.
You locked yourself in. Good luck.
-mr. bill