The FBI issued Seeking Information alerts on its website for Adnan el-Shukrijumah, Abderraouf Jdey, Karim el-Mejjati and Zubayr al-Rimi.
The alerts, signed by FBI Director Robert Mueller, said the men "should be considered armed and dangerous". But he added, "We don't have a specific threat."
The warnings were issued on Friday, a week before the second anniversary of the hijacked plane attacks that took place on 11 September, 2001.
The alerts came the day after the Department of Homeland Security issued an advisory warning that al-Qaida appeared to still be planning hijackings or attacks against the US.
An intelligence official said the FBI alerts were related to the Homeland Security advisory, and authorities believed the men could be involved in some plot involving aviation.
Very serious threat
In March, the FBI made urgent calls for public help to find el-Shukrijumah, a Saudi-born man who they said could pose a "very, very serious" threat as part of an al-Qaida plot against the US. A material witness warrant has been issued for el-Shukrijumah in Virginia.
The original information leading to el-Shukrijumah was based on debriefings of al-Qaida captives. An FBI official said the latest alerts were prompted by fresh intelligence information about the four men.
The FBI said el-Shukrijumah carries a Guyanese passport but may attempt to enter the United States with a Saudi, Canadian or Trinidadian passport.
Al-Rimi is from Saudi Arabia, and al-Mejjati was born in Morocco and has a passport issued in France. His last known entry into the United States was between 1997 and 1999.
Warnings of air attacks come on
eve of September 11 anniversary
Armed and dangerous
El-Mejjati was among the first suspects named in Morocco as possible plotters of May 16 suicide bombings in Casablanca which killed 45 people.
He is described by security services as armed and dangerous. Acquaintances in Morocco said he came from a prosperous family from Rabat and is married to a US citizen.
Jdey, who was born in Tunisia, may be using a passport issued in Montreal and was the subject of a previous alert, the FBI said.
Thursday's advisory said the department had no specific information on individual targets or dates of an attack. It repeated warnings issued during the summer about possible al-Qaida attacks involving commercial aircraft.
The FBI would not say if the four men it was seeking had ties to al-Qaida, although in the past law enforcement has linked el-Shukrijumah with the group.