start from 0000-0000 to 9999-9999, there will be 100,000,000 in total
for number start with 0, => 0000-0000 to 0999-9999 there will be 10,000,000
for number start with 1, => 1000-0000 to 1999-9999 there will be 10,000,000
for number start with 4, => 4000-0000 to 4999-9999 there will be 10,000,000
thus, total available = (100 - 30) x 10^6 = 70 x 10^6