Wakshaani has since answered on the errata thread. You either add a modular limb, whether full or partial, to an actual body part, or you add a modular connector, then add a regular limb.
So in the given example, you'd have a hand with 4 capacity and a lower arm with 10 (- what the connector takes), but the last bit is correct at 12, unless you pick a connector on your body, taking you an extra 30% essence, but giving you the full 15 instead.
At least, that's how I understood it now.
Enhancements for all sublimbs are added together, and the same limits apply.