dict1={1:[1,11,111],2:[2,22,222]}
dict2={3:[3,33,333],4:[4,44,444]}

{1:[1,11,111],2:[2,22,222],3:[3,33,333],4:[4,44,444]}

dictMerged1=dict(dict1.items()+dict2.items())

dictMerged2=dict(dict1, **dict2)

dictMerged=dict1.copy()
dictMerged.update(dict2)

dictMerged=dict(dict1)
dictMerged.update(dict2)

$ python -m timeit -s 'dict1=dict2=dict((i,i) for i in range(100))' 'dictMerged1=dict(dict1.items()+dict2.items())'
  10000 loops, best of 3: 20.7 usec per loop
$ python -m timeit -s 'dict1=dict2=dict((i,i) for i in range(100))' 'dictMerged2=dict(dict1,**dict2)'
  100000 loops, best of 3: 6.94 usec per loop
$ python -m timeit -s 'dict1=dict2=dict((i,i) for i in range(100))' 'dictMerged3=dict(dict1)' 'dictMerged3.update(dict2)'
  100000 loops, best of 3: 7.09 usec per loop
$ python -m timeit -s 'dict1=dict2=dict((i,i) for i in range(100))' 'dictMerged4=dict1.copy()' 'dictMerged4.update(dict2)'
  100000 loops, best of 3: 6.73 usec per loop