Use of http form post

Honestly, your best bet is to actually use $_POST to process this logic. In your form, your submit buttons lack a name attribute which would allow you to do this:

<form method="post" action="http://xxx.yyy/wp-admin/admin-post.php">
    <td width="60" height="26">
       <input type="submit" value="Lot #" name="lot">
    </td>
    <td width="154"         >
       <input type="submit" value="Name" name="name">
    </td>       
    <td width="154">
       <input type="submit"  value="Subdivision - Blk / Lot" name="subdivision">
    </td>
    <td width="96">
       <input type="submit" value="Address" name="address">
    </td>
</form>

And since the only real difference between the <td> elements is the value of data you can eliminate the two hidden fields.

Then in your handler you can use if statements to check $_POST to see which submit button was clicked with the isset() function:

if ( isset( $_POST['lot'] ) {
    $data = 1;
} elseif ( isset( $_POST['name'] ) ) {
    $data = 2;
} elseif ( isset( $_POST['subdivision'] ) ) {
    $data = 3;
} elseif ( isset ($_POST['address] ) ) {
    $data = 4;
} else {
    die( 'Invalid Selection' );
}

// Rest of your handler goes here