public partial class Form1 : Form
    {
        public Form1()
        {

        }

        private void button1_Click(object sender, EventArgs e)
        {
            Form2 f2 = new Form2(this);
            this.Hide();
            f2.Show();            
        }

        public string getString()
        {
            StringBuilder sb = new StringBuilder();
            sb.Append(textBox1.Text);
            sb.Append(textBox2.Text);
            sb.Append(textBox3.Text);
            sb.Append(textBox4.Text);
            return sb.ToString();            
        }
    }

    
public partial class Form2 : Form
    {
        private Form1 _f1;
        public Form2(Form1 f1)
        {           
            _f1 = f1;
        }

        private void Form2_Load(object sender, EventArgs e)
        {
            richTextBox1.Text = _f1.getString();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            _f1.Show();
            this.Close();
        }
    }