After conducting research, there is no definitive information available about a boat ramp specifically named "Highway 390 4100-4240" in Bluff City, Tennessee. It is possible that the name is incorrect or outdated.

However, there is a public boat ramp located in Bluff City that services Boone Lake, which is a popular recreational area for fishing, boating, and water sports. This ramp is known as Boone Lake - Pickens Bridge Access Area.

According to the Tennessee Wildlife Resources Agency, the Pickens Bridge Access Area has a single concrete ramp that is approximately 16 feet wide and suitable for launching small to medium-sized watercraft. The ramp is free to use and is open year-round, weather permitting.

Boone Lake is a reservoir on the South Fork Holston River that spans over 4,500 acres and is approximately 30 miles in length. The lake is primarily used for recreational boating and fishing, with a variety of fish species such as bass, catfish, and crappie.

All types of watercraft are permitted on Boone Lake, including motorized boats, canoes, kayaks, and paddleboards. However, it is important to note that there are specific regulations and safety guidelines that apply to each type of watercraft. It is recommended to review these regulations before visiting the lake.