Python - cortar un string en python

a=['1:[1,2,3,4]', '2:[5,6,7,8]']

print(a[0][3:-1].split(",")+a[1][3:-1].split(","))

import re

raw = ['1:[1,2,3,4]', '2:[5,6,7,8]', '3:[9,10,11,12,13]', '4:[14]']

nums = list(

    map(

        lambda v: re.sub(r'.*\[(.*)\]', r'\1', v).split(','),

        raw

    )

)

numsFlatten = sum(nums, [])

print('Lista de listas')

print(nums)

print('Lista unificada')

print(numsFlatten)

Lista de listas

[['1', '2', '3', '4'], ['5', '6', '7', '8'], ['9', '10', '11', '12', '13'], ['14']]

Lista unificada

['1', '2', '3', '4', '5', '6', '7', '8', '9', '10', '11', '12', '13', '14']