That is'nt going to work: the TextBox2 reference can't be static, so you would have to specify which instance of Form4 you are talking about. Since they are all created dynamically in memory you can't easily search for them, particularly as Windows itself does not know the instances name - it works with the Window Handle - which you can get from the Window Title, but not from the instance name.
The instance name I am talking about here is:
Form4 f4a = new Form4();
Form4 f4b = new Form4();
You can't even access they instances via the names f4a and f4b outside the method that created them unless they are class level variables, and even then you need you know which instance of the parent class you are referring to.
What are you trying to do, that you think this is the best way to proceed? There may be a better way.