c can be used on lists (and not only on vectors):
# you have
l1 = list(2, 3)
l2 = list(4)
# you want
list(2, 3, 4)
[[1]]
[1] 2
[[2]]
[1] 3
[[3]]
[1] 4
# you can do
c(l1, l2)
[[1]]
[1] 2
[[2]]
[1] 3
[[3]]
[1] 4
If you have a list of lists, you can do it (perhaps) more comfortably with do.call, eg:
do.call(c, list(l1, l2))