The theme of home and abroad is explored in The Sign of Four by providing contrasting views of England and India; England is the familiar, and India is juxtaposed as the fantastic, a place where violent, exciting things happen. These things, however, are not compatible with the world in England.
When Mary's father, Captain Morstan, finds out about the treasure in India along with Major Sholto, it sets up a series of violent events that lead to the deaths of several people. The treasure that people are seeking is what drives the violence. In fact, Major Sholto's son is killed after he finds the treasure.
The treasure is from India; the people and characters from India are set up as violent, unreasonable, and fantastic—completely in contrast to the calm, reasonable society at home. In fact, most of the characters from India are the ones who commit violence. Jonathan Small claims that he tried to stop one of the Indian men with a claim to the treasure from killing Major Sholto's son. Sholto himself isn't responsible for the death of Morstan; instead, he was only covering up an accident. The characters from the different places are contrasted to create a theme of safety versus exploration and the real versus the fantastic. This also demonstrates the racist and imperialist mindset of Victorian English society, in which Doyle was writing.
Another way the two places are contrasted is by the home of Dr. Thaddeus Sholto. When Holmes and Watson enter, they notice that it is an exotic place filled with the trappings of India. These things create a place where Dr. Sholto can hide from the outside world (England). This sets up another contrast. Characters can't live both in English society and in the exotic world of countries abroad. Trying to do so has made Sholto's life less English and, therefore, less recognizable to Watson and Holmes.
The treasure also represents the notion of abroad. It is exotic but also somehow wrong. It is exciting but not English. As the detectives attempt to find the treasure and give half of it to Mary, the deaths surrounding it only make it even more clear how wrong it is.
Ultimately, the exotic nature of life abroad is rejected by the more normalized nature of life in England. This is shown when the treasure that Sholto and Morstan brought home from India is thrown into the water and lost. Something that fantastic and unbelievable can't exist at home in England. Because it's gone, Mary is able to marry Watson and move on from the unreal possibilities offered by India.