repeat multiple NULL in R

NULL has no length:

> length(NULL)
[1] 0

So you can't really insert it into a vector. You can either have NA in you vectors or have a list with NULL items.

vec.all = c(rep(NA, 4), rep(2, 4), rep(4, 4))

list.all = c(rep(list(NULL), 4), rep(list(2), 4), rep(list(4), 4))