vb.net string contains only 4 digit numbers(or a year)

how can i check if a string only contains 4 digit numbers ( or a year )

i tried this

 Dim rgx As New Regex("^/d{4}")      
    Dim number As String = "0000"
    Console.WriteLine(rgx.IsMatch(number)) // true
    number = "000a"
    Console.WriteLine(rgx.IsMatch(number)) // false
    number = "000"
    Console.WriteLine(rgx.IsMatch(number)) //false
    number = "00000"
    Console.WriteLine(rgx.IsMatch(number))  // true <<< :(

this returns false when its less than 4 or at characters but not at more than 4

thanks!