s = '''col1, col2, col3, col4
5, 1, foo1, foo10
5, 2, foo2, foo20
6, 1, bar2, bar30
'''
res = {}
heads = ('col3', 'col4')
for line in s.splitlines():
g1, g2, *vals = line.split(', ')
res.setdefault(g1, {})[g2] = dict(zip(heads, vals))
print(res)
cyA9ICcnJ2NvbDEsIGNvbDIsIGNvbDMsIGNvbDQKNSwgMSwgZm9vMSwgZm9vMTAKNSwgMiwgZm9vMiwgZm9vMjAKNiwgMSwgYmFyMiwgYmFyMzAKJycnCnJlcyA9IHt9CmhlYWRzID0gKCdjb2wzJywgJ2NvbDQnKQoKZm9yIGxpbmUgaW4gcy5zcGxpdGxpbmVzKCk6CiAgICBnMSwgZzIsICp2YWxzID0gbGluZS5zcGxpdCgnLCAnKQogICAgcmVzLnNldGRlZmF1bHQoZzEsIHt9KVtnMl0gPSBkaWN0KHppcChoZWFkcywgdmFscykpCgpwcmludChyZXMp
{'col1': {'col2': {'col3': 'col3', 'col4': 'col4'}}, '5': {'1': {'col3': 'foo1', 'col4': 'foo10'}, '2': {'col3': 'foo2', 'col4': 'foo20'}}, '6': {'1': {'col3': 'bar2', 'col4': 'bar30'}}}