R Sum every k columns in matrix

Question

I have a matrix temp1 (dimensions Nx16) (generally, NxM)

I would like to sum every k columns in each row to one value.

Here is what I got to so far:

Answer 1

Another possibility:

x1<-sapply(1:(ncol(temp1)/4),function(x){rowSums(temp1[,1:4+(x-1)*4])})

## check
x0<-cbind(rowSums(temp1[,c(1:4)]), rowSums(temp1[,c(5:8)]), rowSums(temp1[,c(9:12)]), rowSums(temp1[,c(13:16)]))
identical(x1,x0)
# TRUE