Most males achieve their greatest growth spurts by 17, but in my case I grew another 10" over the next year and a half despite being short for my age up until then. Its apparently entirely genetic and hormonal and other then making sure you eat right and get enough sleep there isn't much you can do. People also tend to experience a modest growth spurt around 25 and a few can have one in their 30s even.
no way of telling. my last one was 1/2 inch right after i turned 21.
Honestly, there's no real definition. Four inches in two months sounds like what I'd call a growth spurt. There are cases were people grow a huge amount in a short time, but that's relatively rare. As for when they happen... honestly, throughout puberty. I've known people who grew the most around 14, and others who got an extra nine inches at 18. you just have to wait and see.