Hmm, its always best to keep a consistent extension length across the system. However you can have differing extension lengths even if they share the same first three numbers by using partitions. The problem that you would have then though is if you had range 1xx and 1xxx, if you were on a phone that had a CSS looking to the partitions containing 1xx first and 1xxx second you would never be able to dial the second partition as the callmanager will dial the first match. You would have to use predial numbers to get around the problem.
So if you were in the 1xx range, you would have to dial something like 71xxx to get to the other 4 digit range and use translation patterns to strip the 7 and route the call into the correct partition.
But as I said at the beginning, best stick with a consistent extension length and as you already have 4 digits best stay there.
