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Lava has a pretty huge radius, although Pyrotheum does not seem to (in the interview Iskandar mentioned you can't tweak temp for fluids). Torches do a little, I hear most machines (running or not) and the smeltery.
My monster trap is two 1.5 deep (using slabs) trenches with a 2x2 corridor with a ceiling in between. It gets really crowded with mobs constantly phasing through. For lightning I use exclusively glowstone nooks since they don't raise heat. I got a mob in the corridor about twice.
After I got to the point where I was no longer racing the clock with water and food, I started doing daily backups. I consider the initial early game solved and have no intention of repeating because some mobs phased through the walls of my mob trap and killed me (it's pretty crowded in there)...
Since we're talking about 4 items at a time, too many item types to specify, I find it highly likely the purpose is automating Ex Nihilo ore gravels, sands and dusts with Forestry Packagers.
In which case, the MFR Auto-Packager.