To find b1, we need to use the formula for calculating the sum of the first n terms of an arithmetic series:

S(n) = n/2 * (a1 + an)

Given that S4 = 909 and the common difference g = 10, we know that n = 4 and an = a1 + 3g.

Substitute the values into the formula:

909 = 4/2 (a1 + a1 + 3g)
909 = 2 (2a1 + 30)
909 = 4a1 + 60
849 = 4a1
a1 = 849 / 4
a1 = 212.25

Therefore, b1 = 212.25.