In Jainism, time is divided into cycles called Aara (also spelled as "Ara" or "Aara"), and each Chovisi (set of 24 Tirthankaras) appears in specific time cycles.

If your question is about the Tirthankara of the 4th Aara:

• According to Jain cosmology, we are currently in the fifth Aara (Dusamā-susamā) of the present time cycle (Avasarpini) in Bharat Kshetra.
• The 24 Tirthankaras, including Lord Mahavira, appeared during the third and fourth Aara.
• The fourth Aara (Sushama - Sushama) is known as a period of great happiness, during which many Tirthankaras were born.

However, Jain texts do not specifically assign a single Tirthankara to the "4th Aara." Rather, all 24 Tirthankaras manifested during the third and fourth Aara. The names of the 24 Tirthankaras are:

1. Rishabhanatha (Adinath)
2. Ajitanatha
3. Sambhavanatha
4. Abhinandananatha
5. Sumatinatha
6. Padmaprabha
7. Suparshvanatha
8. Chandraprabha
9. Pushpadanta (Suvidhinatha)
10. Shitalanatha
11. Shreyansanatha
12. Vasupujya
13. Vimalanatha
14. Anantanatha
15. Dharmanatha
16. Shantinatha
17. Kunthunatha
18. Aranatha
19. Mallinatha
20. Munisuvrata
21. Naminatha
22. Neminatha
23. Parshvanatha
24. Mahavira

Summary: There is no single "Tirthankara of the 4th Aara"—rather, all 24 Tirthankaras appeared during the 3rd and 4th Aara according to Jain belief.