The Vanar-King Sugriva sent his forces in all four directions. But after it was confirmed that Sita was in the South, what happened to the Vanaras which travelled in other directions ? Did they rejoin with the forces in the south ?
In Sarga 47 of Kishkinda Kanda of Valmiki Ramayana,the search of vanaras who went to north,west and east is described.
Those monkeys have thoroughly searched at lakes, in the pens of creepers on riverbanks, in vacant lands and in crowded cities and at impassable places with rivers and mountains. [4-47-2] All the monkey commanders explicitly commanded by Sugreeva, very carefully searched all the provinces inclusive of mountains, woodlands, and thick forests of those provinces. [4-47-3]
Later we learn how Vanaras return to Sugriva after being unsuccessful in finding Sita and what they said to him.
Counting the day of their starting as day one, those elephantine monkeys returned to Mt. Prasavana within one month, hopeless of Seetha, and met Sugreeva, the king of monkeys, who is with Rama awaiting the return of monkeys. [4-47-6] Cmdr. Vinata, the great-mighty vanara, came back after searching eastern quarter as Sugreeva had advised, but without finding Seetha there. [4-47-7] Even the valiant and matchless monkey Shatabali explored whole of the northern direction along with his army, but he too came back as Seetha is undiscovered there. [4-47-8] Sushena, the father-in-law of Sugreeva, searching whole of western quarter along with vanara-s, but not finding Seetha there, he came back when one month is completed, and approached Sugreeva. [4-47-9] All of the expeditionists have reached and venerated Sugreeva who is sitting along with Rama on the peak of Mt. Prasavana and spoke this to him. [4-47-10]
They describe how they searched for Sita but were not successful.
All of the mountains, all of the serried forests, all of the riversides up to the edge of oceans, likewise, all of the inhabitations have been searched. [4-47-11] "Those caves that were explicitly extolled by you, they are all thoroughly searched, and immense shrubby hutches that are thickly entwined with climber plants have also been rummaged. [4-47-12] "Search is conducted in the compacted, lopsided, and impenetrable provinces, and colossal beings are hunted out and killed presuming that Ravana might have assumed those odd shapes, and the impassable provinces are searched high and low, time and again. [4-47-13]
They were certain that Hanuman will be successful as He is mighty and went to the direction in which Sita was taken away.
"Hanuma is high-born and exceptionally mighty, oh, Sugreeva, the king of vanara-s, he will ascertain about Maithili, because the son of Air-god, Hanuma, resorted to the direction in which Seetha is taken away." Thus, monkey chiefs reported to their king Sugreeva. [4-47-14]