Speeding up partially known plaintext preimage recovery attack on MD5
It sounds like you're pretty much out of luck.

Yes, there are a few tricks you could use to speed up a brute force search; for example, if we fix everything other than DWORD 3 of B, we can precompute everything up to round 10 of MD5, and compute what the value of the internal 'b' variable of round 52 of MD5 must be to generate the expected hash; this reduces the total number of rounds we compute to 42 (plus a few extra checks that we occasionally do if 'b' happens to be the value we expect, plus redoing the precomputation when we've gone through all possible values of DWORD 3 of B, and we need to try another value for DWORD 1,2,4).

However, such tricks don't come anywhere close to giving you a practical solution.  In fact, even if we were to optimize the time to test a value of 'B' to one CPU clock cycle, and ran the test on one billion CPUs, find a value of B that gives the specified hash would take an expected time considerably longer than the age of the universe.

What you would need is to have a cryptographical weakness in MD5 which would allow you to find such a value without doing an exhaustive search.  Yes, there are a number of weaknesses known with MD5; there's none that gives you this.