format text in textbox

I have 1 text box with 8 digits (always 8 digits only)

eg: "12345678" (this is a string no int but always a numeric string because some time its 00000001 also)

now I just wan to show it as 1234 5678

I mean after 4 digits I want to enter a space

that's all I want Please help.

Pradeep Yadav (Social MSDN)