وَالْأَرْكَانُ مِنَ الْمَذْكُورَاتِ أَرْبَعَةٌ: الْقِيَامُ وَالْقِرَاءَةُ وَالرُّكُوعُ وَالسُّجُودُ
The matn identifies four agreed internal pillars: standing, recitation, bowing and prostration. The final sitting is obligatory and is discussed later, but the source itself records disagreement over whether to classify it as an original pillar or a condition/additional pillar.