Forms in C# are instances of classes themselves. So you could make 4 variables public and set their values before you close the form or switch back to the other form.
public string txtbox1;
public string txtbox2;
public string txtbox3;
public string txtbox4;
Wherever you call this form from.
form2 f2 = new form2();
if (f2.ShowDialog() == DialogResult.OK)
//to get variables
f2.txtbox1; //etc... to retrive values
If this is unclear please let me know I will try to explain further.