First, it depends on how you use it. L4 is pretty close to a "libOS". You can built sandboxes on it, or you can build something more like a monolithic OS on it.

Second, in IoT devices, sandboxing is a lot less interesting, because there aren't that many use cases for sandboxed sensor inputs (you're not RFing or button-pushing whole PDF documents).

I like L4! A lot! But I would be very wary of an IoT device claiming to have inherited security from it.