Book 4 which represents the westernmost thrust of Indo-Aryan expansion during the period of Sudās' descendants Sahadeva and Somaka and the battle "beyond the Sarayu" (IV.30.18) in Afghanistan.
IV.42.8-9 and IV.38.1 may be a late interpolation/addtion. (Talageri 2008).
Rivers and Places
Eastern regions in Book 4: 4. ibha-1. 16. hastin-14. 18. mahiṣa-11. 21. gaura-8. 58. gaura-2.
Western: Book 4: 30. Sindhu-12, Sarayu-18. 43. Rasā-6. 54. Sindhu-6. 55. Sindhu-3.
- Book 4 alone (in Early and Middle RV), names three Western rivers in four hymns, two of which, ironically, have been classified by Oldenberg as late hymns in this Book) (Talageri 2008)
Central rivers: 22. Paruṣṇī-2. 28. (sapta+sindhu-1). 30. Vipāś-11.