Great choice on the 4X10. You're covered for a good majority of situations (as long as you go with good a good wattage head). And good choice on going with 8 ohms.
My advice would be to find a Gallien-Krueger 700Rbii, or a 1001RBii. The difference is the 700RBii has 500 watts, and then 1001RBii has 700. The 1001s are usually in the classifieds in the $450-550 range. These amps are used by a lot of the metal guys. They're rock solid, give a good gritty tone if needed, and sound great with a simple grit pedal in front of it if you need more. My main setup for a good while was simply a Sansamp RBI into my 1001RBii.
Right now, I've moved up to a GK MB800 Fusion and my tone comes from a Darkglass B7K pedal.
Next piece of advice, I would go as HIGH in the wattage as you can afford. You want to save yourself any issues down the road. If your friends are playing with combo amps, you'll be fine. But when they get to half stacks, you'll be competing with 100 watt heads. 1 is enough. Competing with 2 means you'll have to really dial in your eq to push through the mix. But you've got the capability of adding another cab to compete if necessary.
Aim for 3X the wattage that your guitarists are pushing. To compete with a 100 watt half stack, the rule of thumb would mean you'd need about 300 watts to be relevant.
This is why I suggest going as high on the wattage as you can afford. If you get a 500 watt head, you're 500watts if you're at 4 ohms. If you're only connected through one cab @ 8ohms, it's going to only push about 300-350 watts.
So to sum up, my advice is to 1) Go as high in the wattage as you can afford, 2) I'd highly suggest a GK 700/1001, 3) think about what you plan on doing with this band. The 4X10 is a great choice, you've got options to simply add another cab if you really want (to take full advantage of all of the wattage @ 4 ohms.)
