A friend and I ate at Cuatro last night, I didn't see a thread about it, so I thought I'd add my 2¢.
We were there on a Tuesday night, and it was not very crowded, so I can imagine that the atmosphere and attitude of the room could change somewhat, but the room while feeling like it had the potential to be "clubby" (edgy?) was instead relaxed and comfortable, and the staff seemed genuinely happy to have us there.
Also, you'll have to bear with me, it wasn't until about halfway through the meal that I was like "hey this place is pretty good, too bad I don't have my notebook for LTH" - I promise I'll be better next time.

fortunately they have
their menu online so I'll crib from that to jog my memory.
The waitress was very knowledgeable about the specials, and the cocktails were pretty good (although my friend ordered the acerola - black cherry+rum - and they did not have the fixins for that, so she got the guayaba - guava+rum - instead) I had the 10 cane classic which our server warned me "was basically just rum, just so you know" but I wanted to try the 10 cane rum their drink menu was based around - not that I'm a rum aficionado, but it seemed like a good time and place to start. I followed that up (later) with the daiquiri (a "real" one - not a fruit smoothie with a touch of rum - my thanks go out to Alton for allowing me to defend my drink choice against the questioning glances of my guest by citing Hemmingway's appreciation for it) both were quite good (as was the sip of the guayaba I had) and large/strong enough to get the job done.
We shared the coctel vuelve a la vida - a seafood cocktail with tuna, squid, shrimp and octopus mixed in cocktail sauce with avocado. There was nothing spectacular about it, but the plantain chips served with it were pretty good, so we asked for some more ($2 for a plate full of chips - and well worth it). I also had soup - the crema de choclo - corn and potato chowder. I think this was my favorite part of my meal - the flavors were rich and complex, with the sweetness of the corn, the smokiness from the roasting, the starch from the potato surrounded by a rich, creamy base with just a bit of spice*. The flavors were layered fantastically, and when I shared the soup with my friend, I could watch her experience all of them. Good soup!
I had the double pork chop for my entree and sweet christ this was a slab of pig flesh. It was a good four inches deep (sorry, not having brought my notebook, I also forgot my tape measure - next time! next time!) but moist and tender throughout. The plantain mole it was served with was a good accent, but unremarkable from other mole I've had. The yuca cake was also simply inoffensive. The green beans were delicious - served bound in a bunch and stood on end, the thick sofrito sauce "plastered" to the outside of the bunch. The beans themselves were crisp, fresh and sweet, and the sofrito was a nice, spicy accent.
My guest had the special (which is of course not listed on the online menu) - something involving beef medallions or something (which were good) and a plantain/pork cake (which was not so good) and some corn (which was reportedly good - by the time I was to the sharing portion of the meal, it had been pretty well taken care of)
We forewent dessert (I gave up sweets for lent - next time!) and on our way out we ran into a favorite professor who remarked "there are few good restaurants on the south side, this is one of them!" I'd say all in all, I'd have to agree.
Cuatro
2030 south wabash avenue
chicago, illinois 60616
valet parking Wed-Sat
drink specials:
mondays - half-price on select wines by the bottle
wednesdays - 5 dollar caipirinhas
friday after work social - half price appetizers from 3 to 6 pm
sundays - 4 dollar forbidden fruit sangrias and 3 dollar beer, draft and bottles
*edit: I know people's tolerance and tastes for spiciness varies, so I wanted to add that I have a pretty high tolerance for spice, having worked at a place called Habanero for a couple years back in my youth, and I would rate the soup around a jalapeno level of spice, if that.