a = #a random string with unknown length

splitted = len(a) / 4

#codes here that will let me split a into "splitted" part ?
ex:
a = 'aaaabbbbccccdddd'
splitted = len(a) / 4 #which will = 4
#codes here that are gonna split "a" into splitted (4) parts and assign that to b as a list
b = ["aaaa", "bbbb", "cccc", "dddd"]