The woman told police the incident happened while the boy was sitting in the back seat.
A local police spokesman said that the round went through her back after she was shot through the seat.
The boy was unharmed and his mother - named as Jamie Gilt, 31 - is said to be in a stable condition after being taken to hospital.
Police said a .45 calibre handgun was on the floor of the truck, the Florida Times-Union reported.
A public Facebook account for a woman named Jamie Gilt who lives in Jacksonville features pro-gun messages  and has another page called "Jamie Gilt for Gun Sense."
The paper said it was not clear if the Facebook account belonged to the woman who was shot on Tuesday or if the four-year-old referred to in a post about target shooting is her son.
A four-year old boy accidentally shot and injured his mother in their car in northern Florida, US media report.