The equation you have provided is correct:
 A(t)=P(1+r/n)^nt
 Where P = principal amount
  r = interest rate
  n = number of times the amount is compounded in a year
  t = time in years
 The value of n is 12 since it is compounded monthly.
 Substituting the amount to the formula:
 A(t) = $300 ( 1 + 4%/12)^(12)(8)
 A(t) = $300 (1 + 0.003333)^96
 A(t) = $300 (1.376351)
 A(t) = $412.91