After Jatayu's fight with Ravana, lord Rama meets Jatayu and Jatayu explains the event happened telling the direction (south) in which Ravana taken away Mata Sita. So it is well known from Jatayu where Ravana gone. then
Why did Sugriva need to send his forces in all the four directions to search Sita mata instead of one direction as Jatayu already gave information about the direction of south i.e Lanka?
As you defined in you question, Jatayu told Rama that Ravana went towards South.
Even Sugriva himself saw Sita:
त्यज शोकम् महाबाहो ताम् कान्ताम् आनयामि ते | ४-६-८
अनुमानात् तु जानामि मैथिली सा न संशयः |
ह्रियमाणा मया दृष्टा रक्षसा रौउद्र कर्मणा || ४-६-९
क्रोशन्ती राम रामेति लक्ष्मणेति च विस्वरम् |Give-up your lamentation, oh, great-shouldered one, for I will bring back that lady for you. And by inference I know her... undoubtedly I have seen Maithilee while that demon with cruel deeds was abducting her... then she was crying piteously calling 'Rama, Rama...' and also calling 'Lakshmana...'
आत्मना पञ्चमम् माम् हि दृष्ट्वा शैल तले स्थितम् | उत्तरीयम् तया त्यक्तम् शुभानि आभरणानि च || ४-६-११
Myself being the fifth one on the cliff of the mountain, on seeing me she dropped the upper cloth of her sari along with auspicious jewellery. (Sugriva was fifth, others might have been Hanuman, Nala, Neela etc..) [Kishkindha Kanda sarga sarga 6-11 ]
Then why Sugriva didn't try to fight with Ravana at that time?
1 Ravana was Vali's friend.
2 Hanuman was not aware of his powers due to curse by sages (as described in Uttara kanda). And this is the very reason Hanuman didn't fight with Vali for Sugriva. Later Jambavan reminded Hanuman about his powers.
Coming back to your question,
Why Vanaras were sent in all directions instead of South only?
The search operation could not happen soon after Sita's abduction. After Bali's death Sugriva became king and he let rainy season pass without any progress towards Sita's finding. (Reasons for this might be rainy season was not suitable for search operation and Sugriva was waiting for vanaras arrival whom he called from other places). Later Lakshmana went in Kishkindha city and reminded him about his promise. Then search operation started.
So it was possible that Ravana could change the place in meantime (rainy season). And Ravan has many places as he concurred many kingdoms. His relatives, followers were in all directions:
न जाने निलयम् तस्य सर्वथा पाप रक्षसः |
सामर्थ्यम् विक्रमम् वा अपि दौष्कुलेयस्य वा कुलम् || ४-७-२Unfamiliar is the residence, or capability, or valour, or lineage of that sinning demon from a sinister dynasty, in any wise... [Kishkindha Kanda sarga 7-2]
This should be noted that even though vanaras were sent in all directions, more preference was given to South. Most powerful vanaras such as Nila, Jambavan, Hanuman and Angada were sent to South.
And after getting inputs from Jatayu, Sugriva etc., Lord Shri Rama knew that more probability of finding Sita is in South direction thats why he gave his ring to Hanuman.
अनेन त्वाम् हरिश्रेष्ठ चिह्नेन जनकाअत्मजा |
मत् सकाशात् अनुप्राप्तम् अनुद्विग्ना अनुपश्यति || ४-४४-१३By this emblematic ring, oh, best monkey, Janaka's daughter Seetha identifies you to have reached her from my proximity, without apprehension. [Kishkindha kanda sarga 44-13]
VALMIKI RAMAYANA:BOOK3:CantoLXIX.
'Fierce Rávan, king of giant race,Stole Sítá from thy dwelling-place.He calls his magic art to aidWith wind and cloud and gloomy shade.When in the fight my power was spentMy wearied wings he cleft and rent.Then round the dame his arms he threw,And to the southern region flewO Raghu's son. I gasp for breath,My swimming sight is dim in death.Even now before my vision pass Bright trees of with hair of grass, Brings on the thief a flood of woeThe giant in his haste
Jatayu didn't mentioned sri lanka he mentioned southwards.
I am making it cear now:—
23b, 24a. tasya = its - Mt. Mahendra's; apare paare = on the other, shore; shata yojana visR^itaH = hundred, yojana, in breadth; maanuSaiH = by humans; a gamyaH = not, passable; diiptaH = a dazzling; dviipaH = island - is there; tam samantataH maargadhvam = that, till its fringes, you have to search.
"There is a dazzling island on the other side of the shore of Mt. Mahendra, which is breadthwise a hundred yojana-s, and which is an impassable one for humans, and you have to search that island up to its fringes. [4-41-23b, 24a]
25.tatra = there; siitaa visheSataH = Seetha, especially; sarva aatmanaa = anywise; maargitavyaa = shall be searched; saH deshaH tu = that, place, but it is; vadhyasya = killable one; dur aatmanaH = vile-minded one; raakSasa adhipateH = demon, king's; sahasra akSa sama dyuteH = Thousand-eyed Indra, equal, one in resplendence; such a; raavaNasya = Ravana's; vaasaH = dwelling.
"Anywise Seetha is searchable especially there on that island because it is the dwelling place of the king of demons Ravana, who is the coequal of Indra in his resplendence and which vile- minded demon is to be rooted out. [4-41-24b, 25]
so, you can see sugriva knew it but please visit this website for clarification because another verse says he does not know.— http://valmikiramayan.net/utf8/kish/sarga41/kishkindha_41_frame.htm
MY THINKING
Sugriva ordred his army to go southwards to lanka then also it is not sure where he hidden so no chance can be taken.
