Can a male kitten be ually mature at the age of 4 months?

I would like to rephrase my question earlier. I asked if my 4-month old female kitten can get pregnant. In fact I should ask if the 4-month old male kitten can make his sister pregnant. They don't get to go outside as we live in an apartment. My other two dogs have long been neutered. The male kitten is the only threat to his sister. I still think of them as babies. The information I gathered all point to the age of 6-8 months before they reach ual maturity and it is advised that males are neutered at 6 months.