For a traveler, Zuhr, Asr and Isha are performed as 2 rak'ahs.
The source calls Qasr an Azimah — the binding rule for the traveler — not a free choice between 2 and 4.
If a traveler deliberately performs 4 rak'ahs:
• if the person sat for the required sitting after 2, the first 2 count as Fard and the extra 2 become voluntary, while the delay of Salaam is blameworthy;
• if that sitting after 2 was not performed, the Fard is not valid — unless a valid residence intention was formed on rising for the third, as the source separately notes.