Difference between 4 statements - PLease Explain

Question

Hi all... Please Explain........

What is the difference between -

1. Page.User.Identity.Name

2. HttpContext.Current.User.Identity.Name.ToString();

3. System.Security.Principal.WindowsIdentity.GetCurrent().Name.ToString();

4.System.Environment.UserName;
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Which statement is to be used in which scenario ??
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Please explain.

Please give some links/ examples/ explanation...

I have googled it, but no clear idea till date...