stop永远不会在代码中重置为false。每次单击button1时，使用新的CancellationToken可能会更好：

private CancellationTokenSource cancellationTokenSource;

private void button1_Click(object sender,EventArgs e)
{
    // create a new CancellationTokenSource and Token for this event
    cancellationTokenSource = new CancellationTokenSource();
    var cancellationToken = cancellationTokenSource.Token;

    string filename = @"temp1.txt";
    int n = 5;
    foreach (var line in File.ReadLines(filename).AsParallel().WithDegreeOfParallelism(n))
    {
        textBox1.Text = line;

        // Check if token has had a requested cancellation.
        if (cancellationToken.IsCancellationRequested)
            break;
    }
}

private void button4_Click(object sender,EventArgs e)
{
    if (cancellationTokenSource != null)
        cancellationTokenSource.Cancel();
}

您的代码中的问题是无法将stop重置为false。

将stop = false;移出循环（不执行任何操作），并将其放在button1_Click中循环外的任何位置。