How do you do a countdown?

Question

For example, when JAXA launches a rocket, or people count the seconds to the New Year, do they use し or よん to count 4? do they use しち or なな to count 7?

Answer 1

Usually when you count, you use よん for '4' and なな for '7'. That applies to when you count down.

じゅう　きゅう　はち　なな　ろく　ご　よん　さん　に　いち

However, in some situations, pronouncing a certain sequence of numbers became so frequent end became a fixed expression. In that case, '4' and '7' may be pronounced し and しち, respectively. That includes a situation of counting up:

いち　に　さん　し　ご　ろく　しち　はち　きゅう　じゅう