Please read the following table in HTML format from a webpage and then answer the question in detail according to the table. Each table cell is defined by a <td> and a </td> tag. Each table row starts with a <tr> and ends with a </tr> tag. 'rowspan=m' and 'colspan=n' represents a cell spaning over 'm' rows and 'n' columns.

Webpage title:
List of metropolitan areas in the European Union by GDP
Table title:
2018
Table:
<table id='hittask' contenteditable='False'>
<tr> <th colspan=1 rowspan=1 > Rank </th><th colspan=1 rowspan=1 > Metro Area </th><th colspan=1 rowspan=1 > Gross Metropolitan Product (in billions EUR) </th></tr>
<tr> <td colspan=1 rowspan=1 > 1 </td><td colspan=1 rowspan=1 > France Paris metropolitan area, France </td><td colspan=1 rowspan=1 > 681 </td></tr>
<tr> <td colspan=1 rowspan=1 > 2 </td><td colspan=1 rowspan=1 > United Kingdom Greater London, United Kingdom </td><td colspan=1 rowspan=1 > 665 </td></tr>
<tr> <td colspan=1 rowspan=1 > 3 </td><td colspan=1 rowspan=1 > ItalyMilan metropolitan area, Italy </td><td colspan=1 rowspan=1 > 594 </td></tr>
<tr> <td class="highlighted"  colspan=1 rowspan=1 > 4= </td><td class="highlighted"  colspan=1 rowspan=1 > Germany Rhine-Ruhr, Germany </td><td colspan=1 rowspan=1 > 310 </td></tr>
<tr> <td colspan=1 rowspan=1 > 4= </td><td colspan=1 rowspan=1 > Spain Madrid Metropolitan Area, Spain </td><td colspan=1 rowspan=1 > 310 </td></tr>
<tr> <td colspan=1 rowspan=1 > 6 </td><td colspan=1 rowspan=1 > Italy Rome metropolitan area, Italy </td><td colspan=1 rowspan=1 > 297 </td></tr>
<tr> <td colspan=1 rowspan=1 > 7 </td><td colspan=1 rowspan=1 > Italy Turin metropolitan area, Italy </td><td colspan=1 rowspan=1 > 280 </td></tr>
<tr> <td colspan=1 rowspan=1 > 8 </td><td colspan=1 rowspan=1 > Netherlands Randstad, Netherlands </td><td colspan=1 rowspan=1 > 263 </td></tr>
<tr> <td colspan=1 rowspan=1 > 9 </td><td colspan=1 rowspan=1 > Germany Frankfurt/Rhine-Main, Germany </td><td colspan=1 rowspan=1 > 173 </td></tr>
<tr> <td colspan=1 rowspan=1 > 10 </td><td colspan=1 rowspan=1 > Italy Metropolitan City of Bologna, Italy </td><td colspan=1 rowspan=1 > 170 </td></tr>
<tr> <td colspan=1 rowspan=1 > 11 </td><td colspan=1 rowspan=1 > Italy Metropolitan City of Florence, Italy </td><td colspan=1 rowspan=1 > 162 </td></tr>
<tr> <td colspan=1 rowspan=1 > 12 </td><td colspan=1 rowspan=1 > Germany Munich Region, Germany </td><td colspan=1 rowspan=1 > 147 </td></tr>
<tr> <td colspan=1 rowspan=1 > 13 </td><td colspan=1 rowspan=1 > Germany Berlin Metropolitan Region, Germany </td><td colspan=1 rowspan=1 > 146 </td></tr>
<tr> <td colspan=1 rowspan=1 > 14 </td><td colspan=1 rowspan=1 > Italy Metropolitan City of Naples, Italy </td><td colspan=1 rowspan=1 > 140 </td></tr>
<tr> <td colspan=1 rowspan=1 > 15 </td><td colspan=1 rowspan=1 > Spain Barcelona metropolitan area, Spain </td><td colspan=1 rowspan=1 > 137 </td></tr>
<tr> <td colspan=1 rowspan=1 > 16 </td><td colspan=1 rowspan=1 > Belgium Brussels Capital Region, Belgium </td><td colspan=1 rowspan=1 > 133 </td></tr>
<tr> <td colspan=1 rowspan=1 > 17 </td><td colspan=1 rowspan=1 > Germany Hamburg Metropolitan Region, Germany </td><td colspan=1 rowspan=1 > 132 </td></tr>
<tr> <td colspan=1 rowspan=1 > 18 </td><td colspan=1 rowspan=1 > Sweden Stockholm metropolitan area, Sweden </td><td colspan=1 rowspan=1 > 131 </td></tr>
<tr> <td colspan=1 rowspan=1 > 19 </td><td colspan=1 rowspan=1 > Germany Stuttgart Region, Germany </td><td colspan=1 rowspan=1 > 120 </td></tr>
<tr> <td colspan=1 rowspan=1 > 20 </td><td colspan=1 rowspan=1 > Austria Vienna metropolitan area, Austria </td><td colspan=1 rowspan=1 > 111 </td></tr>
<tr> <td colspan=1 rowspan=1 > 21 </td><td colspan=1 rowspan=1 > Denmark Greater Copenhagen, Denmark </td><td colspan=1 rowspan=1 > 102 </td></tr>
<tr> <td colspan=1 rowspan=1 > 22 </td><td colspan=1 rowspan=1 > Norway Greater Oslo Region, Norway </td><td colspan=1 rowspan=1 > 96 </td></tr>
<tr> <td colspan=1 rowspan=1 > 23 </td><td colspan=1 rowspan=1 > Greece Athens metropolitan area, Greece </td><td colspan=1 rowspan=1 > 95 </td></tr>
<tr> <td colspan=1 rowspan=1 > 24 </td><td colspan=1 rowspan=1 > Italy Metropolitan City of Bolzano, Italy </td><td colspan=1 rowspan=1 > 92 </td></tr>
<tr> <td colspan=1 rowspan=1 > 25 </td><td colspan=1 rowspan=1 > Republic of Ireland Dublin Metropolitan Area, Ireland </td><td colspan=1 rowspan=1 > 84 </td></tr>
<tr> <td colspan=1 rowspan=1 > 26 </td><td colspan=1 rowspan=1 > Finland Greater Helsinki, Finland </td><td colspan=1 rowspan=1 > 75 </td></tr>
<tr> <td colspan=1 rowspan=1 > 27= </td><td colspan=1 rowspan=1 > France Lyon metropolitan area, France </td><td colspan=1 rowspan=1 > 74 </td></tr>
<tr> <td colspan=1 rowspan=1 > 27= </td><td colspan=1 rowspan=1 > United Kingdom Greater Manchester, United Kingdom </td><td colspan=1 rowspan=1 > 74 </td></tr>
<tr> <td colspan=1 rowspan=1 > 29 </td><td colspan=1 rowspan=1 > France European Metropolis of Lille, France </td><td colspan=1 rowspan=1 > 72 </td></tr>
</table>
Question:
How did Rhine-Ruhr's GDP rank in the European Union?
Answer:
The Rhine-Ruhr metropolitan region was the 4th largest GDP metropolitan area in the European Union.