firstbodega, asking people to bring chairs isn't an offense, but I'd borrow before I'd ask. big_city_beth, four people is absolutely doable without a dining table, but you have to stop thinking of it as a seated dinner party and start thinking of it as a cocktail-party plate situation.
First, seating: if your sofa seats two comfortably, you only need two more spots. One can be your desk chair; the other can be a low folding stool or a milk crate with a cushion. The key is to have everyone at roughly the same height, otherwise the person on the step stool will be eating dinner in another climate zone. A firm bench seat or a stack of two crates with a folded blanket works better than a low footstool.
Second, surfaces: do not try to use the coffee table as the main table for four plates. It will become a knee-knocking mess. Instead set the kitchen counter or a dresser top as a serve-yourself buffet. Put plates, napkins, forks, glasses, and the food in one line. People fill plates and then return to their seats. A tray under the coffee table can hold one or two glasses, but it shouldn't be the centerpiece.
Third, the menu: skip anything requiring a knife. Bowl food is your friend. Big grain salad, a sturdy pasta, braised beans, or a curry over rice. Add a spoon, a fork, and a wedge of bread on the side. That means every guest has one hand free for a glass, and balancing is much easier. For four, I'd do two mains max, maybe one salad, one dessert you can eat with your fingers.
The real mistake people make is buying folding chairs and then trying to create a circle of four around a table that doesn't exist. Borrow first, then decide if you'll actually use them more than twice a year before you spend money or give up the space.