The best way to add a horizontal line between rows is with a CSS borders. First, you want to collapse all the borders of the table so that there is no space between the cells (this might help your solution as well, but I don't recommend using hr for this purpose). Next, specify a border on the bottom-side of each cell (td). You can similarly put borders to the left, right, up, etc. See the self-contained HTML below.
<html>
<head>
    <style type='text/css'>
        table.test { border-collapse: collapse; }
        table.test td { border-bottom: 1px solid black; }
    </style>
</head>
<body>
    <table class='test'>
        <tr><td>1</td><td>2</td><td>3</td><td>4</td></tr>
        <tr><td>1</td><td>2</td><td>3</td><td>4</td></tr>
        <tr><td>1</td><td>2</td><td>3</td><td>4</td></tr>
    </table>
</body>
</html>
For more border options, check this w3Schools page.