Currently, it seems as though four of your elements can do any of the following in their domain (Though let me know if I have assumed anything I shouldn't have):
- Create
- Modify
- Destroy
Compared to "Entropy" - which can only "Destroy", but from all four of the other elements. I think they may be more powerful than you give them credit for - so here are some examples of what they may be capable of:
Heat Mage: Can redistribute heat, or make things hotter, or colder. It sounds lame, but it's really cool! Other than the obvious abilities of frying or freezing people/objects, and keeping himself at a comfortable temperature, he may be able to cause massive weather changes if he's got the area-of-effect to do so - possibly causing sudden fog, rain, tornadoes, etc. Heat is energy, and heat mages could pump energy into, or suck it out of, just about anything.
Light Mage: Create light where there is none, or create darkness where there is light. Where this mage gets complicated is where the boundaries of "light" are. A Light Mage might be capable of controlling not just "visible" light, but all of the Electromagnetic spectrum! This means they can easily communicate with other Light mages via radio waves, cook their food (or enemies) relatively quickly with microwaves, and - with high levels of control, can modify light to bend around objects - or reflect objects that aren't there! In other words, invisibility and illusions. Oh, and lasers. Burn a hole through something at the speed of light? Sounds good.
Electricity Mage: Aside from obviously tasing anything, by controlling the movement of electrons you can control magnetic fields. Think "Magneto" from "X-Men" - pretty much capable of "controlling" metal. In a medieval world, this means you can stop weapon attacks - assuming the weapons aren't wooden. In a modern world, this means a lot opportunities to fry, wipe, and destroy electronics. You'd also be able to melt (or weld if you stop early) just about anything.
Force Mage: Consider that the forces in control by a force mage are what control pressure, sound, the state of matter(and can manipulate it), and are the causes of devastating natural events such as earthquakes and tsunamis. A force mage could instantly blow an enemy's eardrum, followed by vibrating a single point into an explosion while shattering the earth and turning it to liquid.
Compared to the others, you may consider Entropic magic to be a mechanism of Time. Given time, everything decays, "fires go out,stone turns to dust, metal corrodes, wood and flesh rot away at its touch". As such, its uniqueness might come from being able to stop Entropy, which would be like stopping time - but maybe you can speed it up as well, which allows you to move and act faster - but age faster at the same time.
Without any restrictions on all of the elements so far, there is no "balance" at this point. All the magic is capable of ending the world. You'll have to scale how "hard" it is to do each different type of magic according to how you want your world to be.