Underside of Lionel No.4 showing one of the two cast iron weights. Its likely that the weights were designed to add traction. A similar weight is affixed to the opposite end of the locomotive. I have also seen these weights mounted over the frame inside the cab.

Part of the Bild-A-Loco collector shoe is also visible. A feature of this design is that the collector shoe can easily be removed. With the collector shoe removed the axles with their captive bushings and pressed on wheels can be lifted out of the motor bearing plates.

Lionel No.4 Bild-A-Loco. Grey with Apple green hatches. This example does not exhibit an Apple green stripe along the lower edge of the cab as is described in popular guide books. There are two cast iron frame weights.

Ives No 3242 looking brand new.

Lionel No.4. Reverse switch is part of the motor assembly. The lever protrudes through an arc shaped cutout on the cab.

Lionel No. 4U Outfit.
Unassembled O-Gauge No.4 Bild-A-Loco locomotive. Assembled and decorated locomotive cab and frame, motor parts, wheels, drive gears arranged on a metal tray and circle of track.
The black enameled motor base permits the locomotive motor to be converted to a stationary motor. The axles and flanged wheels nay be exchanged with a flywheel-pulley shaft or a capstan-sprocket shaft. The stationary motor drive shafts are visible to the left and right of the brass locomotive collector shoe assembly. The stationary motor is known as the Bild-a-Motor.
The No. 4U is only 4U while in the box. When assembled it becomes a No.4 An example of a piece that must have the box. This outfit appears to be missing the screw driver and include an extra flywheel pully shaft.

Ives No. 3243 Wide Gauge locomotive.

The Lionel No. 254 in yellow. Rubber stamping "YELLOW" is visible on the box. The No. has the reverse switch horizontally mounted on the frame with the lever protruding through the engin cab.