In general, there are two ways to clear a textbox - using your case
textBox4.Clear();
or
textBox4.Text = string.Empty;
this issue is I'm not sure of your logic/program flow, and thence 'where' to do this in your code
[edit] I guess you do it before this/the new read "
Quote:
but problem if i read data for new qr code
happens, but you havnt shown much 'code' so I dont know where that is in your program [/edit]