I don't know if this is the most efficient (is speed or memory the top concern?) solution:
library(data.table)
x <- data.table(col1=c(rep("hello,how,are,you",3),
"Lirum, Larum, Löffel, Stiel"),
col2=1:4)
x[, ind := .I]
x[, col3 := strsplit(col1, ",", fixed=TRUE)]
x[, col4 := col3[[1]][col2], by=ind]
# col1 col2 ind col3 col4
# 1: hello,how,are,you 1 1 hello,how,are,you hello
# 2: hello,how,are,you 2 2 hello,how,are,you how
# 3: hello,how,are,you 3 3 hello,how,are,you are
# 4: Lirum, Larum, Löffel, Stiel 4 4 Lirum, Larum, Löffel, Stiel Stiel