Answer: 3,2,4,1
Explanation:
The Muslims first conquered Egypt in 641 CE and over the next 4 decades continued to conquer North Africa until they had overrun all of North Africa.
It was after these conquests that they started hearing about civilizations South of the Sahara through the conquered people and began to visit and trade with them.
This trade flourished and brought great riches to the nations that were involved. The man arguably considered the richest in history, Mansa Musa, was King of the Mali kingdom that benefitted richly from such trade.
Along with that trade came Islamic teachings and through this medium many were converted. Mansa Musa himself was a Muslim who's riches became well known when he performed the Hajj to Mecca.
Islam then slowly spread to the south of Africa after the trade gave them a foothold in the West.
So, the correct order is,
Arabs come into North Africa, then All of North Africa comes under Muslim
rule, then The gold-salt trade flourishes on the trans-Saharan route and finally Islam spreads to western and southern of Africa.