Looking at apartments 4 1/2 3 1/2 ect.. confused?

The 1/2 refers to the bathroom, regardless whether it's a half-bath or not. Usually if there is more than one bathroom, it will specify "2 bath, 3 bath" etc. So the 3 1/2 in this ad would be 3 rooms and one bathroom. The 3 is actually refering to One bedroom, One kitchen, and One living room, and not 3 bedrooms. A 4 1/2 would have 2 bedrooms, one kitchen, one living room, and the half is the bathroom again.