The width is stored in the spline’s control point. You can have a different width per control point or set them all to the same width.
If you already have a spline laid out, select your spline. In the Details panel, click the Control Points button next to “Select all connected” so all of the control points are selected for that spline. Under the ‘Landscape Spline Control Point’ section, there is a Width field. Set that to 4000 to get 4 meters. If you have multiple splines, repeat these steps for all of them.
this is also mentioned in the docs: Landscape Splines | Unreal Engine Documentation