I've been doing a lot of 4-man PvP lately, so I'm going to put a list of my observations here for people who don't do it very often. If you've ever done RA or TA enough to earn a gladiator point or two you probably wouldn't learn anything from this, so you can just skip it.
Most of the professions are pretty well-represented in the arenas, but not necessarily for good reason. A pair of assassins and a pair of monks generally does pretty well, if they're coordinated. In any other team, there should only be one Sin, as they require a disproportionate amount of Monky attention. Rits tend to lose horribly if the enemy team has anyone capable of interrupts but otherwise more then earn their keep, and Rangers are a disproportionately common choice of profession. Axe warriors tend to fare better then sword wielders, and hammer-ladies are a little luckier then them. A Nec who relies on tons of vampiric skills and who creates a Flesh Golem out of the first player to die is a pretty common occurance, so if you face a Nec be aware that they should die first and won't go down easy. Ele's are pretty rare in TA (and RA for that matter) anymore, because they tend not to work very well.
Focusing fire on one enemy is a much better strategy here then in most other PvP areas, because very few non-warrs can withstand it even if their team has a monk. If there are two monks focusing on one (but not using your best attacks) and then switching to the other after 3-5 seconds usually does the trick.
When you have killed an enemy, especially a monk, don't feel bad about camping their corpse. If you have any interrupts you can switch targets, but otherwise wait for one of their teammates to use a res sig on them. Well-armored enemies who don't rely on energy (basically Warrs) aren't worth waiting for, but any spellcaster comes back with 25% energy (which you knew already) and even less health then before. After they're dead a second time there's a much lower possibility that they'll be res'ed (especially if you're standing on their corpse) so it's best just to move on.
AoE spells are generally less effective because there are less people for them to target, and map-specific advantages (height, flag-controlled dohickies, and Res Priests) are extremely helpful. Speaking of which, the strategy for the lightning/fire towers that your flag controls is important. Getting there first doesn't matter, so having a Warr or Ranger run to the flagstand is a bad idea. A player who doesn't attack, but uses other skills (Monk, Mes, Rit, Ele, etc.) should carry the flag to the stand but not place it into the stand. If the other team doesn't have anyone carrying their flag yours may be safely placed, but if (as is usually the case) an enemy Warr or Ranger has the flag, you just stand there and use your skills/spells which he holds his flag and waits for you to place yours so he can steal control. If you're lucky, that player will waste their time, if they drop their flag to attack you (as is often the case) another member of your team should return it for them (you can't, because it causes you to drop yours).
I'd put up some more advice, but my roommate wants to use the internet, so that's it for now.
