Or, as an (untested) alternative.
Select
Distinct RCD.Variable4
FROM Route_Call_Detail RCD
WHERE RCD.[DateTime] > '12/4/10'
AND RCD.[DateTime] < '12/5/10'
AND CASE WHEN SUBSTRING(RCD.Variable4, 1, 6) NOT LIKE '%[^0-9]%' THEN CONVERT(INT, SUBSTRING(RCD.Variable4, 1, 6)) ELSE NULL END BETWEEN BETWEEN 5011 AND 7999
)
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability