AS mentioned in a comment to your Question, you can use the following:
SomeString = Mid(StringName, Start, Length)
Putting that into a Sub will look as follows:
Dim OriginalString As String
Dim String1 As String
Dim String2 As String
Dim String3 As String
Dim String4 As String
OriginalString = 309701406
'Mid wants a String input
'Because of the need to split the Numerical String it is not explicity required to have the " " around then String input line.
'Should you want to Split a String of Alphabetical Characters the " " will definitely be required.
String1 = Mid(OriginalString, 1, 2)
String2 = Mid(OriginalString, 3, 3)
String3 = Mid(OriginalString, 6, 3)
String4 = Mid(OriginalString, 9, 1)