If I were to tell the truth to my boyfriend, he would kill me! 

但是，为什么不能说

If I told the truth to my boyfriend, he would kill me! 

再比如：

If he were to graduate on schedule, you could call me blind. 

为什么不能说：

If he graduated on schedule, you could call me blind.

    【答】 if ... were to do 是表示对将来的虚拟的形式之一，不是只能用这种形式，也可以用过去时。所以，你的四个句子都是正确的。

     《刘毅英语语法宝典》p.363有以下内容，可以解答你的问题：

    If 从句用过去时，可同时表示与现在事实相反，或与将来事实相反的假设，通常可由上下文的文意来辨认，但若要明确地表示所涉及的时间，可加进时间副词，就可一目了然。

     If you were happy today, you'd make others happy.如果你今天快乐，你也会使别人快乐。

     If you were happy next week, you'd make others happy.如果你下星期快乐，你也会使别人快乐。

     If your father knew this, he would be angry. 如果你父亲知道这件事，他会生气的。【与上两句相同，可同时表示与现在和将来事实相反的假设】