On 4 Horse the "front" two bosses are melee and will follow you around. The "back" two are casters that stand in their corners and cast on anyone with the highest threat that is in their corners. Of course, as you know, if no one is in the back corners (or within a reasonable distance of the front two) for a few seconds, there will be a judgement that kills everyone instantly. Also, the front left boss will drop a meteor on someone within 30 yards that splits its damage across everyone within 5 yards. It does like 35K damage, so this will wipe any toon or most pairs of toons standing by themselves.

So the basic strategy is have seven characters (in 10-man) standing stacked up on the front left and use everything they have to burn down that boss before they get too many stacks. A self healing, high armor class tanks the front right boss solo and another healer stands in the back and heals both himself and another character standing in the back on the other side. When the first boss goes down, everyone swithces. Front left 7 go to front right, front right goes to right back, and back left and right swap positions. From there everyone switches till the bosses are down, making sure there is a tanking class on the front right till he goes down (since he is melee). So that's the basics (which you may have already known, but it was quick to type up).

In thinking about solo boxing this, if you have a priest available the back portion of the fight should be easier. If the two back characters stand fairly close to each other just behind the raised platform (about 30 yards away from their bosses and less than 30 yards away from each other), then prayer of mending is fabulous. Cast it once and it constantly bounces back and forth, doing most of the healing you need on the back two all by itself. Keep casting it when it's timer is up and it makes the healing on the back two much easier. You would probably also need two macros to heal the healer and the other back character that you need to alternate casting while the front boss is being burned down since prayer of mending probably won't keep those two up all by itself. All in all, though, pretty simple. The only issue with this is that the back left boss drops those nasty black void zones wherever the back left character is standing. So this requires some movement to get out of and the area in this boss's corner behind the platform but close enough to get heals from the back right healer will quickly get filled with void zones, so they'll eventually have to move either so far away from the boss towards the healer that everyone gets wiped by the judgement or so close to the boss that they are out of healing range of the back right healer. I haven't figured out how you'd get past the void zone issue with only one healer in the back.

The other thing is that after the front left boss is burned down (and basically for every transition), you have to have four sets of characters changing boss corners almost simultaneously. In the back maybe you could have a button on both toons that targeted each other and autofollowed, then hit autorun, then hit stop after they pass each other and switch zones.

As for Gothik, it seems like splitting up your party into two teams, stacking each in a corner, and setting up most of simple /assist <tank> macro, then random tab targeting with ample use of almost random taunts should get you through the waves. It would basically be how I do Noth - one tank, stacked heals and DPS in a corner with everyone assisting the tank and the tank keeping everything on him because they basically have to run through him to get to anyone else. The only real complication is that you'd be splitting up your team and doing it x2 at once. Since the dead side is harder, I would either put my best tank, heals, and DPS on the dead side and watch that tank for taunting/tab-targeting (hoping the live side party takes care of itself with the randomly timed targetting/taunts), or stick 6 toons on dead side, four on live and taunt/target based on whats going on on the live side. Some sweet spot combination of toon split and which party is getting your attention should work. The Gothik waves only get intensive on the live side at the end, and only for about 20 secs. On the dead side there is perhaps a minute of truly difficult waves.

Anyway, there are some really basic thoughts, maybe they'll help you a bit. I'd love to see you do these, as your successes give me confidence (and proven strategies) to try these fights myself.