Some questions first: where are you in the UK? Which way does the border face, or if you don't know, how much sun does it get? And when you say 3 to 4 feet, is that depth from front to back, or length from side to side? Either way, need to know the depth and length.

If you have a look under G at the bottom of the page, you will find ideas for ground cover plants, if that is what you looking for, rather than gravel or bark etc. Hope this helps.