Already there are 5 peoples in the room.
Now,
After you enter into the room, the total number of persons will be
5 + 1 = 6
Then, you killed 4, so the number of persons alive in the room is 2.

But it is asked to find out the total number of persons regardless of alive or dead.

Hence, the 6 persons will remain inside the room.

Originally it is mentioned that there were 5 people in room and as the killer or I come, It makes 6 people in the room.

So after killing 4 people, no. of people left in the room will be 6, 4 dead and 2 alive.

Answer: 6 persons.

Explanation: 

If there are 5 people in a room, and one walks into the room and kill 4 out of 5 persons, then there will be 6 persons in the room because the person who killed will also be counted as a person in the room, along with those who the person killed. The ones who were killed would not change into anything else or disappear from that room if the one person who killed them has entered the room. So, including that one, there will be 6 persons in that room.