The T4 was first built several years before the Invasion of Naboo. The primary function of the droid was to act as a mobile defense platform to protect dignitaries who were visiting hostile worlds. During the invasion, many were redeployed to Theed to enforce the peace and prevent the queen from fleeing. Even though they failed at the last task, they performed well enough to continue being used up to the early stages of the Clone Wars. They cost 26940 credits new and were restricted to military usage.
The T4 was primarily used in defensive positions, and were often found near crossroads or checkpoints. They could be modified to be artillery spotters, listening posts, or patrolling guard droids.
- Scavenger's Guide to Droids