So if you were to do 4 piece bonus for fire, how would you glyph?
By my quick and dirty napkin chart, it looks like you still only get a natural PoM/AT/Comb twice in a 6 min fight, and unglyphed you would have to stagger PoM to line up every 3rd combustion and glyphed every other combustion. Seems like glyphed is less work, just wait for both to come up, which essentially kinda pushes one combustion off the table I think.
The thing about combustion is not to use them as often as possible but to build up awesome combustions which is now increasingly hard with the CM changes. The 4 set piece isn't actually even that good for fire since it might encourage you to pop a half arsed combustion. The combustion glyph is definately the way to go though.