Extract values from vector and repeat by group

Hi all,

I hope you can help.

I have a data frame 'df':

group=c(rep(1,8),rep(2,10),rep(3,11))
var=rnorm(29)
time=c(seq(1,8),seq(1,10),seq(1,11))
df=data.frame(group,var,time)

I would like to extract the value from 'var' for each 'group' at 'time'=4 and repeat these extracted values in a new vector ('new') n times where n is the number of rows for each group. I did this by hand as below, but there must be a quicker way:

subset=subset(df,df$time==4)
subset
group        var time
4      1  0.2531270    4
12     2 -0.3600128    4
22     3  0.4194730    4

df$new=c(rep(0.2531270,8),rep(-0.3600128,10),rep(0.4194730,11))

Any questions please ask,