Maybe you could use the CurrentCellKeyPress event. In it, identify the cell somehow and check the current length and set the e.Handled to true to ignore the keystroke.
private void gridControl1_CurrentCellKeyPress(object sender, System.Windows.Forms.KeyPressEventArgs e)
{
GridCurrentCell cc = this.gridControl1.CurrentCell;
if(cc.RowIndex == 2 && cc.ColIndex == 3 && cc.Renderer.ControlText.Length >= 4)
{
e.Handled = true;
}
}