The fact that Google chose to build QUIC on top of UDP rather than on top of IP directly shows that not even Google think it's possible to introduce any new Layer 4 protocol.
QUIC operates on top of UDP, which would make it layer 5.
QUIC is the transport layer (layer 4).
> QUIC is an experimental transport layer network protocol [1]
You can run layer 4 on top of layer 4. (Just as you could say, run TCP layer 4 on top of HTTP layer 7.)