Edit: ok, after I re-read the paper a couple of times it is clear that Table 6 is easily derived from Table 5, which describes the collision differential. So the real question is: how did the authors come up with the collision differential?
The paper Cryptanalysis of the Hash Functions MD4 and RIPEMD describes a set of sufficient conditions for collisions of MD4 (Table 6 in the paper). The authors explain how to verify these conditions, but there is no info about how they got them in the first place. I'm curious, what could be a possible procedure?