فَإِنْ صَلَّى أَرْبَعًا، وَقَعَدَ فِي الثَّانِيَةِ مِقْدَارَ التَّشَهُّدِ: أَجْزَأَتْهُ الرَّكْعَتَانِ عَنْ فَرْضِهِ، وَكَانَتِ الْأُخْرَيَانِ لَهُ نَافِلَةً، وَإِنْ لَمْ يَقْعُدْ فِي الثَّانِيَةِ مِقْدَارَ التَّشَهُّدِ فِي الرَّكْعَتَيْنِ الْأُولَيَيْنِ: بَطَلَتْ صَلَاتُهُ.
Translation: If a traveller performs four and sits after the second for the duration of tashahhud, the first two suffice for the obligation and the last two are voluntary. Without that sitting after the first two, the prayer is invalid.
The rule turns on the final sitting: for a traveller whose obligation is two rak‘ahs, the sitting after rak‘ah two is the required final sitting.