If you want to split one 13 digit number into 4 numbers, then surely Split is the way.
If it's just the way for some validation or input taking then I would suggest you to use a masked textbox to take the input.
Mask TextBox ASP.NET Control[
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Enhanced Textbox Control[
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There are few similar control based article here at code project itself.
This sort of control would restrict only given number of digits, no less no more in a specified format. Change it as per your need.