One significant figure.
1 significant figure.
One - the trailing zeros are not significant.
1
4000000
4000000
One. The zeros are just placeholders.
Just the one and it is 4
One - the zeros are placeholders unless indicated.
Without the decimal point, there is only a significant figure.
1 significant figure. It can also be written as 4x10^6 (one sig fig)
One - the four. None of the trailing 0s are significant digits.