Objects in 3rd and 4th Density occupy the same space, as when humans are moved through walls. What you call sub-atomic particles move at different frequencies or vibrations. They do not collide or even recognize each other. Their motion does not put them in the same place at the same time, anymore than if you were to walk through the forest. The trees are more dense in this instance, in that from your view they aren't moving at all. You guide yourself past the trees. Sub-atomic particles in 4th Density in essence flow past those in 3rd Density, as water does to obstacles in its path. Molecules in 3rd Density and 4th Density simply slide by each other, and do not interact. Thus, they can even occupy the same space. Space, in fact, is more empty than full. A planet, likewise, can and is both 3rd and 4th Density at the same time. Regarding the issue of how we travel, or move a human through walls. We are not allowed to explain fully. Suffice it to explain that we make a reservation in the vibrational density we wish to move to, and then move.
If we are moving into a density already occupied, we ensure a safe move by first freezing the objects in the reserved area so that no molecular action is occurring. It is in essence a dead stop, similar to what the soil at landing sites is exhibiting when plants fail to grow there for some time after the landing as the mineral nutrients are frozen for a time. After objects in the reservation area are frozen they are taken into the density we are shifting from, still in their frozen state. The receiving point is essentially in the middle of a vacuum, to minimize molecular encounters. When the frozen portions from the reservation area are at hand, we step into this same area, arrange ourselves around any object temporarily taken from the density we are moving to, and all is then quickly returned to the target density. Horrors such as occurred at the Philadelphia Experiment are not possible with this procedure, for if the target density is so crowded that we cannot step into the arena, we return the objects and select another spot.