The risk can be calculated by using this formula (pos1/total1)/(pos2/total2) where pos1 denotes the number of cases in the first group, pos2 in the second group, and the total variables are the group totals

However, the epitab function of the epitools package, is needed for your problem:

## maybe:
## install.packages("epitools")
library("epitools")

For input formats, try this:

tab <- matrix(c(4,16,40,168),byrow=TRUE,nrow=2)
epitab(tab,method="riskratio")
## $tab
## Outcome
## Predictor Disease1 p0 Disease2 p1 riskratio lower upper
## Exposed1 4 0.2000000 16 0.8000000 1.000000 NA NA
## Exposed2 40 0.1923077 168 0.8076923 1.009615 0.8030206 1.269361
## Outcome
## Predictor p.value
## Exposed1 NA
## Exposed2 1