yonsenman 四千万
Remember that "man" means "ten thousand" (10,000) and over "man", you don't get a new word for numbers until you get through all of "sen man", so how many 10,000s do you have? Look at the first digit in 10,000: in Japanese you would call this "ichi man" because you have one "man". In this case, that digit is a zero, so go to the left. If you only count three to the left, you're still in "sen" territory, so don't change "man" to something else. Therefore, you have 4,000 (yonsen) "mans" (10,000s).
@dagiarrat Thanks for explaining this.
@Larrikin Sorry, I haven't yet implemented conversion from English numbers to Japanese, but it's on the todo list.
Numbers to Japanese numbers
Tried to explain to my Japanese friend how much 40,000,000 dollars was today and couldn't convert it.