You still have almost 2252 different combinations.
Minor correction: If you have 4 possibilities for one character in a 64 character private key, then the total number of possibilities is 2
254. If you knew for certain one character in the key,
then you would have 2
252 possibilities.
Another correction: This address
does have a private key. In fact, it has many. It is just that nobody knows what any of them are (or at least, as far as we know nobody knows what any of them are). That address is generated from a RIPEMD-160 of:
0000000000000000000000000000000000000000
Any private key which produces a public key, which when hashed with SHA-256 then RIPEMD-160 produces the above number, would be able to spend coins at that address.