There be 问题

看白勇的语法辅导书上有这么一道题：

A recent national study ogf the public shools shows that there are now one microcomputer for every thirty-two pupils, four times as many than there were four years ago.

提供的正确答案是：

A recent national study ogf the public shools shows that there is now one microcomputer for every thirty-two pupils, four times as many as there were four years ago.

我的疑问：为什么最后的there were four years ago 不改成there was four years ago? 这样似乎才更对称阿?

请各位不吝赐教，谢谢！

同问。