It seems, according to Patañjali, the Fourth (chaturtha, scil. praaNaayaama) is the conditio sine qua non of dhaaraNaa:
https://www.ashtangayoga.info/philosophy/yoga-sutra/chapter-2/ https://www.ashtangayoga.info/philosophy/yoga-sutra/chapter-2/ बाह्याअभ्यन्तर विषयाक्षेपी चतुर्थः ॥५१॥ https://www.ashtangayoga.info/philosophy/yoga-sutra/chapter-2/item/baahya-aabhyantara-saya-sepii-caturtha/ bāhya-ābhyantara viṣaya-akṣepī caturthaḥ ॥51॥ The fourth pranayama technique ultimately transcends breath retention after exhaling or inhaling. ||51|| https://www.ashtangayoga.info/philosophy/yoga-sutra/chapter-2/item/baahya-aabhyantara-saya-sepii-caturtha/ ततः क्षीयते प्रकाशाअवरणम् ॥५२॥ tataḥ kṣīyate prakāśa-āvaraṇam ॥52॥ The veil covering the light of the true self then vanishes. https://www.ashtangayoga.info/philosophy/yoga-sutra/chapter-2/item/tata-h-k-siiyate-prakaa-sa-aavara-nam-52/ धारणासु च योग्यता मनसः ॥५३॥ https://www.ashtangayoga.info/philosophy/yoga-sutra/chapter-2/item/dhaara-naasu-ca-yogyataa-manasa-h-53/ https://www.ashtangayoga.info/philosophy/yoga-sutra/chapter-2/item/dhaara-naasu-ca-yogyataa-manasa-h-53/dhāraṇāsu ca yogyatā manasaḥ ॥53॥ https://www.ashtangayoga.info/philosophy/yoga-sutra/chapter-2/item/dhaara-naasu-ca-yogyataa-manasa-h-53/ And the mind develops the capacity for harmony with thoughts (dharana). ||53|| https://www.ashtangayoga.info/philosophy/yoga-sutra/chapter-2/item/dhaara-naasu-ca-yogyataa-manasa-h-53/ Perhaps the locative plural (dhaaraNaasu) collectively, so to speak, refers to dhaaraNaa, dhyaana and samaadhi, i.e, saMayma??
