How many significant digits are in 4000000?
At least 1 and at most 7.
It could be 3,999,999.9 rounded to 7 significant figures;
It could be 3,999,999 rounded to 6 significant figures;
It could be 4,000,015 rounded to 5 significant figures;
It could be 4,000,429 rounded to 4 significant figures;
It could be 3,999,999 rounded to 3 significant figures;
It could be 4,049,999 rounded to 2 significant figures;
It could be 4,492,467 rounded to 1 significant figure.