c# show text in textbox in 4 textboxes

You can use the String.Substring Method[^] for this:

string original = textbox1.Text;
textbox2.Text = original.Substring(0, 1);
textbox3.Text = original.Substring(1, 2);
textbox4.Text = original.Substring(3, 2);
textBox4.Text = original.Substring(5, 2);

I would approach a problem like this by, first, creating a small "general purpose" tool that would let me split a string into a bunch of "chunks," where the size of each chunk can vary.

private List<string> ChunkString(string source, params int[] chunksizes)
{
    // check for match between source length and total digits required
    if (source.Length != chunksizes.Sum()) throw new ArgumentException("bad argument");

    List<string> results = new List<string>();

    StringBuilder sb = new StringBuilder(source);

    char[] chunk;

    foreach (int chunksz in chunksizes)
    {
        // create a new Char[] to hold the chunk
        chunk = new char[chunksz];

        // copy #chunksz characters from StringBuilder to chunk
        sb.CopyTo(0, chunk, 0, chunksz);
        // trim #chunksz characters off the StringBuilder
        sb.Remove(0, chunksz);

        // save the current result as string
        results.Add(new string(chunk));
    }

    return results;
}

Then, in your case, I might use it this like this:

// in some method or EventHandler
List<string> results = ChunkString("2145576", 1, 2, 2, 2);
// assuming you have references to the four TextBoxes in a TextBox Array named 'tbAry:

TextBox[] tbAry = new TextBox[]{textBox1,textBox2,textBox3,textBox4};

for (int i = 0; i < tbAry.Length; i++)
{
    tbAry[i].Text = results[i];
}

Notes:

1. While I use a 'StringBuilder here to try and minimize the creation of new Strings, I'm not at all sure that has a real "payoff" here ... the business of grabbing Char[] arrays and turning them into strings may be offsetting any gains using a 'StringBuilder.

2. I chose to avoid using Linq here, but you could probably whip-up some Linq code to do this ... which looks simple until you try to understand what it does :)