Loveland Police Department Lt. Jeff Pyle says the accident occurred just after noon Saturday.
A witness told authorities that a driver in a Toyota Corolla was driving erratically when the car went off the road and careened into the picnic area, striking the victims as they sat at a table.
All four were taken to the hospital with non-life threatening injuries.
The unidentified driver was taken into custody. Pyle says alcohol or drugs are suspected to have played a role in the accident.