Hi,
The task explicitly assumes that m1 does not start a new round trip.
> If m1 does not start a new round-trip from B's perspective
Therefore, some previous message started the round trip from B's perspective.
This message is not shown in the image, as it is a hypothetical question.
This means that the keys for m1 and m3 are derived from the symmetric ratchet.
The next round trip starts when the next share from A is received (contained in m2) to compute the new DH key.
M2 is received directly before m4 is sent.
Therefore, B starts a new round trip with m4.
Cheers,
Sebastian