It's very simple. list(1:4) creates a single element(vector) whereas list(1,2,3,4) creates multiple elements(vector)
> l
[[1]]
[1] 1 2 3 4 5
> l1
[[1]]
[1] 1
[[2]]
[1] 2
[[3]]
[1] 3
[[4]]
[1] 4
[[5]]
[1] 5
list(1:4) is a atomic element whreas list(1,2,3,4) is not atomic.