Whenever the Fantastic Four go into combat, they have specialist uniforms required for their mutation. Throughout the released films, there has been many different re-designs.
The uniforms were originally designed by Victor Von Doom for their project into space. The synthetics acted as a second layer of skin, therfore adapting to the body's individual needs.
When the cosmic storm hit, their uniforms became exposed, therefore adapting to the individual's mutation.
Reed Richards later discovered that the synthetics in the uniform adapted to their needs, so he adapted the designs and produced a uniform for each member of the team. Ben Grimm, however, was unable to wear the whole uniform due to his increase in size and strength, so only wore half of the uniform at the suggestion of Sue, since the material could stretch a little.
The team wear the same uniforms as before. Also Johnny Storm endorses a new uniform with a lot of promotions by leading retailers. However, none were taken on, subsequently never producing or using the uniforms beyond Johnny showing it off.
The team's uniforms are containment and beta suits, designed to stabilize and control each individual's mutation. Johnny's being made of fire-proof materials, Sue's being made of materials that can turn invisible with her, and Reed's being designed to keep him together. Ben is the only member who doesn't receive a uniform due to the extremity of his own mutation.