Chapter 4 occurs in the same setting that Chapter 3 does, and is merely a continuation of that previous chapter but after Edmond and Mercedes have left the location. In Chapter 3, the characters meet Caderousse and Danglars drinking and join them at a table outside an inn, as the following quote from Chapter 3 demonstrates:
The young man stopped suddenly, looked around him, and perceived Caderousse sitting at table with Danglars, under an arbor.
This is where the action of Chapter 4 occurs, as the young men discuss their different reactions and feelings to the forthcoming marriage between Edmond and Mercedes, and how they are going to do their best to try and ruin the chances of happiness that Edmond and Mercedes have out of petty envy and spite. Towards the end of the chapter, the three men leave and it is clear that something has been set in motion by Danglars that he is quite proud of, as he talks in the final line about this stratagem now "effecting its purpose unassisted." The apparently innocent setting of a table outside an inn has been converted into something more sinister.