That's what I get trying to balance rough numbers in my head. I was using roughly the number of atoms, not grams. Here's the math laid bare:

    Frac Remain In 1 Day = 0.5 ^ ((1/365)/4,500,000,000 years)
    Frac Remain In 1 Day = 0.999999999999578
    
    Frac Decay In 1 Day = 1 - Frac Remain In 1 Day
    Frac Decay In 1 Day = 4.22 * 10^-13
    
    Have 4 Decays In 1 Day = Total U238 * Frac Decay In 1 Day
    4 = Total U238 * 4.22 * 10^-13
    4 / (4.22 * 10^-13) = Total U238
    Total U238 = 9.48 * 10 ^ 12 atoms

I stand corrected, thanks for catching that.