How to transfer data between two totally unrelated Forms C#

Glad to be of help. I noticed that I forgot the class so that is below (just for a single field; modify to your needs)

    class myDataClass
    {
        public string txt;

    }

Create a class that can hold the data. Create an public static instance of that class in Program.

When you switch from Form2 to Form4, store the data from Form2 in that instance and next read it back when you load (or init) Form4.

Below code uses 2 forms (1 and 2, not 3 and 4)

    static class Program
    {
        ///

        /// data storage for transfer
        ///

        public static myDataClass theData;

        ///

        /// The main entry point for the application.
        ///

        static void Main()
        {
            // create an object to hold the data of form1
            theData = new myDataClass();
...
...

Yellow indicates the changes to a standard Program.cs file

My Form1 has one textbox textBox1 and a button to show my Form2
The button code

        private void button1_Click(object sender, EventArgs e)
        {
            // store data for transfer
            Program.theData.txt = textBox1.Text;
            // create form2
            Form2 frm2 = new Form2();
            // and show it
            frm2.ShowDialog();
            // get rid of it
            frm2.Dispose();
        }

My Form2 has one textbox textBox2 that will show the data. It also has a load event.
The load event code

        private void Form2_Load(object sender, EventArgs e)
        {
            // copy data from storage to textbox
            textBox2.Text = Program.theData.txt;
        }

Note that there might be other ways, but this is what I would use.