All about comma usag

Hi. 1) Why would you say that the latter two - nos. 3 and 4 - need to have a comma after the words "room" and "meeting"(if it is indeed the case) and the first two (nos. 1 and 2) need not have one after the words "room" and "song"(if it is indeed the case)?

1.To identify the most popular song among the teenagers in this room which method would you use?

2. For the purpose of identifying the most popular song what method would you use?

3.In order to identify the most popular song among the teenagers in this room, which method would you opt to use?

4.If you were there in the meeting, which method would you have thought of using to identify the most popular song among those there?

2) Can you tell me if this doesn't need a comma at the place indicated since it need not be there to allow a moment to breathe for a reader. Frankly, whenever I read the sort of sentences like the one below, I read, not being too much conscious of whether I need to breathe or not in midst of my reading and that doesn't help me greatly when it comes to trying to understand the situations involving this type of comma usage. Come to think of it, I think there would be a greater tendency for me to take a breathing moment if the part precedes took importance contextually.

Three out of the last five years he has shown much enthusiasm in this school activity.

3) Would you say this comma is placed to allow a breathing space?

You will want to your strongest reasor for the last, since it is the last thing you will do.