You can use ggplot2 to give you smoothing line.
libary(ggplot2)
d = data.frame(DPConc= c(0, 83, 166, 416),
DPActivity=c(100, 67.71, 6.3, 16.55))
ggplot(d, aes(x=DPConc, y=DPActivity)) + geom_point(size=3) + geom_smooth() + theme_minimal()
By default that uses a LOESS function to smooth the data points, but here you'll find information about other functions.
As others have said, go to CrossValidated for info on smoothing functions (although they may not be willing to help you interpolate 4 data points.)
