Four from seven leaves three.7减4余3.哪个是主语?

问题描述:

Four from seven leaves three.7减4余3.哪个是主语?
如题
此句的主语应该是数词,那么是哪个数词呢?

我认为是Four from seven
如果补充成It's Four from seven leaves three.
It's 也应该是形式主语
真正的主语应该还是Four from seven这个短语