They four are good friends.  

这个句子是正确的。four为数词，作they的同位语。意义类似于 the four of them. 以下为COCA语料库类似例句：

Our harvest being gotten in, our Governor sent four men on fowling, that so we might after a more special manner rejoice together, after we had gathered the fruit of our labors; they four in one day killed as much fowl as, with a little help beside, served the Company almost a week.

Yet what difference could it possibly make, since they four (he suddenly knew) were the sole survivors, the city above them gone up in smoke and the ashes cooling under the rain.