I have some ideas for when Sgt. Cortez travels to different time period levels for TS4's story. This maybe a dumb idea, but I feel inclined to share it anyway
1953 - Streets of London and Buckingham palace
Storyline takes place during the Queens coronation while setting is in a stereotypical 1950's streets in England during the street parties. This idea is heavily inspired by an episode of Doctor Who ("The Idiot's Lantern").
Various time crystals are discovered by Doctor Seth Graven who intends on using the crystals odd power to create alternative fuel for cars and homes. Many companies invested on his newly discovered fuel, as it is a better, renewable alternative for homes and cars. Towards the Coronation, many people reported strange side effects caused by being surrounded by fumes emitted from cars, power plants etc. Side effects included eyes balls melting, suddenly bursting into flames, arms falling off, big/deformed body parts and many other effects. These side effects are also contagious which is a big problem in various streets of London when many people will gather in street parties for the Coronation.
As expected, disaster happens. Many people turn into feral mutants and zombies. Many lose the ability to reason and will attack on sight. Many London Streets are in chaos. Those who watch the Coronation on TV witnessed a member of the Royal family turn into a hideous mutant. Even the local Bobbys (British Police) and Queen life guards are infected.
Sgt. Cortez arrives on the scene and has to find a way to retrieve the time crystal without exposing himself to the hazardous aura. Angry mutants and zombies stand in his way. He may also have to make a visit to Buckingham palace, as a possible Timesplitter or two maybe present there.