The one who takes a short trip to a city where he has a relative is considered to be a traveler

He is considered to be a traveler so
long as that is not his home, even if he has a relative there such as
a brother, sister, etc. But he should not pray on his own, rather he
should pray with the jamaa’ah and pray four rak’ahs with them, because
it is obligatory to pray with the jamaa’ah. But if there are one or
more others with him, then they may either pray the shortened prayers,
or they may pray with the jamaa’ah in that city and pray the full prayers. 

But if he intends to stay more than four
days, then he has to pray the four-rak’ah prayers in full, whether he
is traveling alone or they are a group.