usage - How do you do a countdown?

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

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