In Python, each module has its own namespace. By default, you need to include the module name when calling a method in that module:

import binascii
num = binascii.a2b_base64("04")
print num

Note that a2b_base64 converts a string of Base64-encoded binary data into its raw binary form, which sounds like it's not what you actually want. To convert a string of hexadecimal digits into a string of hex data, use a2b_hex:

>>> import binascii
>>> binascii.a2b_hex("04")
'\x04'