I wouldn't say people are always stressed, but it does tend to be the kind of place where people like to talk about how stressed they are a lot. It's sort of a family joke, if you will--I have so much work! No I have so much work!--etc etc. Of course there's a grain of truth--there is a lot of work and people do get stressed--but no, people are not always stressed.
4 classes is the normal courseload. You need to take 32 to graduate, so if you have no outside credit you will graduate in 4 years taking 4 classes every semester. However you can take 3 classes without any special permission and it definitely is done, especially among thesis-ing seniors.