Try something like this (code in Form3 class):
Form4 frm4 = new Form4();
frm4.lblN2.Text = this.txtf.Text;
frm4.Show();
Alternative would be to modify constructor method in Form4 to accept string parameter and invoke it as follows:
Form4 frm4 = new Form4(this.txtf.Text);
frm4.Show();