In Yuddha Kanda, Chapter 7, when the ministers are eulogising Ravana about his qualities, they say that the four sons of Varuna were killed (defeated?) by Ravana.
śūrāḥ ca balavantaḥ ca varuṇasya sutā raṇe |
nirjitās te mahābāga catur vidha bala anugāḥ || 6-7-12
Oh, illustrious king! Varuna's sons, valiant strong and who were followed by an army having four limbs (viz. elephants, chariots, cavalry and infantry) were defeated by you in a battle.
Who are those sons and why were they killed by Ravana?