The simplest way to get 5 sections with your particular request is (using the Paint Terrain By Spline Panel script) to first set the Centre WIdth to the overall width you want the outside Grass layer to cover for example 12m
Set the Centre Texture to the required texture and select Paint
You then set the Centre width to the width of your Centre grass texture( i.e 4 m )and the Edge WIdth (i.e.2m) to the distance required for your gravel texture
with the required textures set select paint
This will give you
| -------------------------12m ---------------------------|
| 2m Grass | 2m Gravel | 4m Grass | 2m Gravel | 2m Grass|
When painting multiple textures you should always work from the outside in
Another variation with different Edge Textures
Starting with
1. | 2m GrassGravel | 8m Grass | 2m Concrete |
working with only the centre 6m section
2. |2m Gravel | 4m Asphalt | 2m GrassMoss|
Giving you
| --------------------------------12m--------------------------------------|
| 2m GrassGravel | 2m Gravel | 4m Asphalt | 2m GrassMoss | 2m Concrete |
Edit: This method also works if you are using the Paint Foliage by Spline script.