When running a stored procedure with a parameter, why does 4T give me the same result as 4?

My stored proc worked great when CarNumber was an integer, but I found out that the data may be more than just a number.  It could include letters too!  

Here is the stored proc:

CREATE PROCEDURE dbo.GetDriverInfoSP
@CarNumber as varchar
AS
SELECT
CarNumber,
LastName=rtrim(LastName),
FirstName=rtrim(FirstName),
Equipment,
Engine
FROM
Qualifications
WHERE CarNumber = @CarNumber
GO

CarNumber    LastName     FirstName    Equiptment   Engine
4                | Madison     | Davey       | Prototype 1 | Chevy
4T              | Clark          | Mike         | Prototype 2 | BMW

Datatypes

CarNumber = varchar
LastName = varchar
FirstName  = varchar  
Equiptment = varchar  
Engine = varchar

The following Line in Query Analyser:

GetDriverInfoSP '4T'

gives me the same results that the following line gives me:

GetDriverInfoSP '4'

What gives?