2.3.73 चतुर्थी चाशिष्यायुष्यमद्रभद्रकुशलसुखार्थहितैः

Word by word (padaccheda)

Substitute / term चतुर्थी prathamā · ekavacanaIndeclinable च avyayaRight context आशिषि saptamī · ekavacanaWith / by आयुष्य- मद्र- भद्र- कुशल- सुख- अर्थ- हितैः tṛtīyā · bahuvacana

Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.
Indeclinable An avyaya such as च "and", वा "optionally", न "not".
Right context Saptamī (7th case): "when X follows", the operation applies to what precedes (1.1.66 तस्मिन्निति निर्दिष्टे पूर्वस्य). Also a condition or domain ("in the sense of").
With / by Tṛtīyā (3rd case): accompaniment or means.

Full reading

चतुर्थी च आशिषि आयुष्य-मद्र-भद्र-कुशल-सुख-अर्थ-हितैः

Pāṇini leaves out words that carry over from earlier sūtras. Expand to see the rule as a reader actually understands it.

Commentary

The 4th as well as the 6th-Case-affix may be used, when blessing is intended in connection with the words आयुष 'long life', मद्र 'joy', भद्र 'good fortune', कुशल 'welfare', सुख 'happiness', अर्थ 'prosperity' and हित 'good'.
The च in the sutra makes the employment of Dative optional : in the alternative we have Genitive.

Vart:- In this sutra आयुष्य &c., include their synonyms also.

Thus आयुष्यं देवदत्ताय भूयात् or आयुष्यं देवदत्तस्य भूयात् ॥ Similarly चिरं जीवितं, मद्रं, भद्रं, कुशलं, निरामयं, सुखं, शं, अर्थो, प्रयोजनं, हितं पथ्यं देवदत्ताय or देवदत्तस्य भूयात् ॥

Why do we say when 'benediction is intended'? Observe आयुष्यं देवदत्तस्य तपः 'the austerity is the cause of the long life of Devadatta.' Here there is no option allowed : and the Genitive case is only employed.

Śrīśa Chandra Vasu, The Ashtadhyayi of Panini (1897), via ashtadhyayi.com.