Although the purist will state that there are four forces, when explaining observed phenomena at the nanoscale it is very useful to include interactions such as ionic and covalent bonding, hydrogen bonds, Brownian motion, van der Waals forces, thermal vibration, rotation , adhesive and cohesive forces and subcategories of these interactions.  Often, the effect of what is observed at any scale (macroscale to nanoscale) is dependent upon the priorities of these forces.  For example the interaction between planets is driven by the gravitational force because of the large mass of the objects,  The strength of the interaction of planets due to the electrostatic forces exists, but is very small -- overshadowed by the gravitational forces.  The opposite is often true at the nanoscale, atoms and molecules are significantly impacted by electrostatic forces - and because of the small mass, minimally impacted by gravitational attraction.  So it is just a matter of which force or interaction is the top interaction for any given situation.  These modules use many different activities which allow students to evaluate the priority of different forces and interactions with different materials and at different scales.