[KirillPanov]

Layer 4 is the lowest layer of the OSI model that isn't packet-based (i.e. can handle messages of arbitrary size).

IMHO that's the highest layer of the OSI model that is useful. After that it's various ways of tunnelling streams inside of streams.

[fanf2]

That’s not true, though: the OSI model has connection-oriented services at the lower layers too.