Chemistry - Hottest region of Bunsen flame
The question itself is poorly written as given.
The diagram should have "regions" and "points."
So what seems to be labeled "Region 2" and "Region 4" I would name as "Point 2" and "Point 3" respectively.
So the innermost conical area would be "Region 1", the next conical area would be "Region 2" and the third conical area would be "Region 3."
Region 1 is where the mostly unburned gas and oxygen mixture are pushing above the lip of the Bunsen Burner. Region 1 exists because the gas coming out of the tube is cool. If the air port is open and the gas flow is too low then the gas will start to burn down the tube and you'll get a "strike back" where the flame is either (1) blown out or (2) burns at the jet. If the gas flow is too great you can blow the burning region off contact with the upper tube. If you increase the gas flow even more then you can in fact blow the flame out.
Region 2 would be a reducing region within the flame. This region is hot and burns the fuel and oxygen coming out of the tube.
Region 3 would be a oxidizing region of the flame. Here oxygen from "outside air" (oxygen which didn't come up the tube) is migrating into the flame to burn the excess fuel which is not bunt in region 2.
You'd use the reducing and oxidizing regions when doing bead tests for identification.
Point 2 would be the hottest part of the flame as shown by the composite image of a paperclip in the flame from the YouTube video.
Region 1 is much cooler because it contains a mixture of gases that it unreacted.
The gases in the tube are traveling too fast for the flame to enter the tube.
The inner cone is this unreacted gas mixture as it exists the tube.