How to append a list when its elements are not defined in the conventional sense

how can I append a list together if the list is composed of values from statements like "for i in a"? consider the example below

a=[1,2,3,4]
for i in a:
    b=2*i
    print([b])

If I want to append all the lists of b in to a single new list, how do I do that?

Thank you!