Share a border with, be next to (4)
I believe the answer is:
abut
I believe this is a double definition.
'share a border with' is the first definition.
(I've seen this in another clue)
'be next to' is the second definition.
(I've seen this before)
(Other definitions for abut that I've seen before include "Meet" , "Lean against the odd tuba" , "Adjoin, press up against" , "Touch, lean on" , "Touch upon" .)