pandas: convert index type in multiindex dataframe

IIUC you need the last level of Multiindex. You could access it with levels:

df1.index.levels[-1].astype(str)

In [584]: df1.index.levels[-1].astype(str)
Out[584]: Index(['1', '2', '3', '4', '96', '99'], dtype="object", name="Values")

EDIT

You could set your inner level with set_levels method of multiIndex:

idx = df1.index
df1.index = df1.index.set_levels([idx.levels[:-1], idx.levels[-1].astype(str)])