4-SEN, or Foursen, was a droid who served Gorry and the Mantellian Separatist Movement on Ord Mantell during the Separatist War. He was stationed in the Separatist base's warehouse at Mannett Point when the smuggler known as Ace broke in and killed a number of soldiers there.
4-SEN was a bipedal droid with masculine programming who was active during the Cold War and lived on the Galactic Republic's planet of Ord Mantell in the Bright Jewel system of the Mid Rim. He belonged to a Human named Gorry who was a member of the Mantellian Separatist Movement, a group who opposed the planet's corrupt government known as Ord Mantell Central Authority on their support of the Galactic Republic and wished to be independent. While 4-SEN worked for them they were engaged in a civil war with the government and the Republic Army.
4-SEN and Gorry were stationed for part of the war on the Separatist-held island of Mannett Point. There they were amongst the soldiers who guarded the Separatist's personnel database which was kept in a captured warehouse on the island. During the time they were there the smuggler known as Ace broke into the warehouse with help from a inhabitant of Mannett point named Reki. The Smuggler intended to download all the data from the database and use it to locate a Separatist named Skavak who had stolen their ship and a shipment of weapons from Rendia Freight. The Smuggler successfully reached the database after killing a number of the guards in the warehouse.
As the Smuggler was downloading the data 4-SEN's sensor's detected the bodies of the guards the smuggler killed and so put the warehouse's alert status on red while informing Gorry. The droid could not however detect any hostiles not realizing that Ace was an intruder. The smuggler asked Gorry when he had last calibrated 4-SEN's sensors and persuaded him they were most likely faulty and so there was no need for an alarm. Gorry agreed and he and 4-SEN moved on allowing the Smuggler to successfully complete their task and escape.
4-SEN is a non-playable character in the video game Star Wars: The Old Republic released by BioWare in 2011. He is a character involved in the smuggler-class storyline appearing in one single mission. It is possible for the player to simply attack and kill Gorry and 4-SEN as a dark side option instead of persuading them to move on.