A parking area for five or more cars serving a nonresidential use shall be screened from an adjoining R district or a ground floor residential use by a solid wall and landscaping six feet in height, and constructed to withstand a 15 pound per square foot wind load, except that the height of a wall adjoining a required front yard in an R district shall be four feet. A carport or open parking area for five or more cars serving a residential use shall be screened from an adjoining lot in an R district or a ground floor residential use by a solid wall or fence six feet in height, except that the height of a wall or fence adjoining a required front yard in an R district shall be three feet.