You can set the global.par knitr-option to TRUE like this:
```{r setup, include=FALSE}
opts_knit$set(global.par = TRUE)
```
Then set par in the next chunk and plot something:
```{r}
par(mfrow=c(5,4))
plot(something)
```
The same par-options will now be passed to the following chunks (until changed again).