Alright, Module 4 is incredibly and totally awesome.
The quests are (so far, haven't done the raid and preraid quest yet) absolutely awesome - very challenging, great scripts, fantastic and fun battles. The new AI makes things incredibly fun, especially going against casters who now happily use a lot of nasty spells like Symbol of Pain to block strategic passages, and archers who are actually kiting you!
The new quests are a great mixture between interesting (!) battles (with archers and casters on ledges, and with a lot of cover), traps that make you go "freakingfuckingshitWHOA!" when you run into them the first time (there is one trap that continuously pulses with shockwaves, sending you flying... into spears and blenders at the ceiling), and really great and fiendish puzzles that still get our head swimming. They also make use of a lot of odd skills - for example, if you go into the Crucible to get beaten into a pulp (to join the Storm's Fist brigade, lead by a sarcastic stonegiant), you should make sure your party has at least one really strong swimmer in the party - there is an AWESOME underwaterpassage that is absolutely deadly and great, and has nothing at all to do with any underwater bits you have ever seen before.
The wilderness/adventure zones are a nice touch. The Cerulean Hills (for level 1-4s) look fantastic and are great to RP in (they learned landscaping from LOTRO, actually), but are a bit low in treasure. The Gianthold Ruins however is a bleak landscape with lava and frozen statues of Giants, Dragons and other monsters locked into an eternal battle - and an army of creatures. They provide lots of experience, and there's about 10 random chests scattered in it.
The monsters: While orcs are nice and quite unique when it comes to their abilities (raging archers are quite nasty, makes me reconsider my plans for a ranger/barbarian), the real winners are the Stone, Cloud and Fallen Giants. Where Firegiants are oversized dwarves, these guys are the real deal.
Level 16 (level 14 quests on elite) loot is quite niceish. Disruptors and other tasties like a +15 hide and a +6 strength glove dropped for me so far. Since the new quests are *really* challenging, a good party that trusts each other and doesn't rush blindly around is an absolute must in most of the quests, especially in the more tricky puzzle passages.
There's also quite a lot of love for rogues. Plenty of quests feature secret areas behind locked doors, the traps basically shout for high reflexes on elite setting - and even Sisila managed to blow her first trap up. We're still laughing at her for that one.
All in all, come back, level up, there's lots of things to do, and they're all deadly and exciting.