Not sure about buying them for credits, but I know you can use 2nd Order Exploration badges to buy them at Earth Spacedock (2nd Floor).. I've been farming Hromi exploration missions to outfit my science vessel with new weapons.
Keep in mind some of the higher weapons have rank limits on them -- I learned this the hard way. When you hover over a MkIV weapon it should say, in green, on the right hand side, the rank required to use that weapon. So don't bother spendign the energy credits (or anything else) until you are, or are close to, that rank.
Exploration points are easily worth it. You get nice exp also and its quite fast. IM harvesting MArk IV gear myself at the moment. You need to be lvl 12 though to get exploration missions for mark IV stuff.
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.