Pressure-Lock Doors: Two sets of stainless steel doors, surrounded with rubber bladders that expand to create an air lock. Often accompanied by a pressure change system which ensures that air can only flow from lower containment to higher containment areas, so it can ultimately be cleaned.
Decontamination Shower: The decontamination shower bombards scientists with water and virus killing chemicals for several minutes before allowing them to move from one area to another.
Clean Air systems: Various systems of ducts and air filters ensure that 99% of all particles larger in size than .0000003 meters in width. This is essential because many pathogens at level four are airborne.
Clean water systems: 1500 gallon tanks collect water that has been used and clean it both chemically and thermally, water is heated to over 250 degrees to kill pathogens.
Nonporous walls: All surfaces the a BSL-4 lab should be completely smooth and have no small spaces for particles to collect. It is common for surfaces to be coated with epoxy to ensure a smooth, easily cleaned surface, and even fire sprinklers are equipped with valves to prevent the travel of particles through the pipes.
Other precautions obviously include protective suits for the workers (including air tanks), very limited access into the lab, and very advanced containments cells for viruses themselves.
These labs are exceedingly expensive to produce, and they are the only situations in which such dangerous disease can be studied. Hopefully labs like this will lead to the cure of today's most dangerous diseases.