数组越界问题

排序问题，最后显示引索数组越界，怎么解决
为什么会有这个问题

{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        float[] a = new float[5];
        float temp;
        int i = 0;
        int j = 0;
        private void bu1_Click(object sender, EventArgs e)
        {
            a[i] = Convert.ToSingle(text1.Text);
            lal.Text += a[i] + " ";
            i++;
        }
        
        private void bu2_Click(object sender, EventArgs e)
        {
            for (i = 1; i <=4; i++)
            { for(j=i+1;j<=4;j++)
                if(a[i]>a[j])
                {
                    temp=a[j];
                    a[j]=a[i];
                    a[j]=temp;
            }
            lal2.Text = "\n 排序之后:";
            for(i=0;i<=4;i++)
            {
                lal2.Text +=a[i];
            }
        }
    }
}