OBJ files can contain polygons with an arbitrary amount of vertices (so yes, those are quadrilaterals). You need to convert them to triangles first if you want to render them using OpenGL.
The easiest and dumb way is to just treat them like a triangle fan, but that assumes all polygons are convex (which is likely the case for most models anyway). If that works out for your data then just do that and don't worry anymore. Otherwise, you can try loading the model in a 3D editor and reexport the model ensuring all faces are turned into triangles (Blender has a checkbox that says "triangulate" if I remember correctly, don't know about other editors).