If you know it only needs the next 4 characters...change your substring line to this:
Dim strsubstring as string = strmystring.substring(i,4)
The second parm on the substring function means you want to pull the next 4 characters.
--- oops...I didn't read your question well enough...
You'll need to pull the whole thing like you did before. I suppose then you could loop through the string and look for the first character that is alpha...you could use the IsNumeric
] function. Or you could figure out a way to do it with regular expressions.
Here are google results
] for regular expressions. Here are CP Articles
] for regular expressions. These should help you get started.