At the beginning of Act 4, the audience discovers that Brutus and Cassius have fled Rome and are "holed up" near the city of Sardis. In Act 4, Scene 3, when word arrives that Antony and Octavius are bound for the city of Philippi, Brutus immediately asks Cassius what he thinks of their advancing toward Philippi to head off an attack from their enemies. Cassius does not think it wise and gives valid reasons for his answer (the trip to Philippi will wear on their men when they could just fortify themselves while Antony and Octavius' troops wear out themselves and their supplies by having to travel a greater distance). Brutus dismisses Cassius's suggestions by stating:
" 'Good reasons must of force give place to better' " (4.3.228).
The "better" that Brutus refers to is, of course, his own strategy to advance toward Philippi. The pro of his suggestion is that if Cassius and he wait for the enemy to approach, Antony and Octavius will be able to gather more troops along the way and restock their supplies because the people in the area between Philippi and Sardis have only tolerated having Brutus and Cassius' camp there and are more supportive of Antony's cause.
Both battle plans make sense, but Cassius' is slightly more logical because at the beginning of Act 4 Brutus and he were bemoaning the fact that they do not have enough men, have not paid them properly, and are short on supplies. An unnecessary march could further hinder the men's ability to fight.
In the end, Brutus' advice is followed, and Cassius and he march to Philippi, seemingly doing Antony's bidding. This conversation and ultimate decision on Brutus's part prove fatal and demonstrate, once again, Brutus's tragic flaw of poor judgment.