I read that you can buy these for energy credits somewhere?
I am federation, and I just got my first real cruiser and it has crap gear on it and I noticed I can't even take down one klingon's shields with 4 beams firing on it.
so.. i figured maybe i need mk iv's
To the OP, if you're firing 4 beams on your cruiser, unless you're in full assault mode with EPS stations, you're going to run out of energy quickly. If you keep firing when you're energy is 0, you'll do squat dmg, which may be what you're seeing. Just because you have 4 slots doesn't mean you can use 4 beams easily.
There are ways to do it, but they're not all that intuitive.