Sebastian's friend gives him 4 special-edition baseball cards to start his own collection. Sebastian then buys 6 new baseball cards for his collection every month. If x is the number of months, what is the linear function, f (x), that represents the total number of baseball cards in Sebastian's collection as a function of the number of months?

A: f(x) = 4x
B: f(x) = 6x
C: f(x) = 4x + 6
D: f(x) = 6x + 4