Comic book fans know that the Flash is one of DC Comics' most powerful heroes, but most probably do not realize that Flash is in fact so powerful that he can easily reproduce the powers of all members of Marvel Comics' Fantastic Four.
Starting with "just" the power of super speed, the Flash's set of abilities slowly but steadily increased, reflecting the character's popularity and the creativity of his creators. Since the introduction of the Speed Force in the 1990s as the source of his powers, it has sometimes appeared that there is no limit to what the Flash can do, as he is able to manipulate some of the fundamental aspects of physics.
The Fantastic Four, on the other hand, have a much "simpler" set of powers, each loosely drawing on one of the classical elements of nature: Earth, Water, Fire and Air. The Thing is strong and durable, Mr. Fantastic is a super-genius who can stretch long distances, the Human Torch can generate large amounts of heat, and the Invisible Woman can manipulate invisible force fields and disappear from view. The Speed Force, however, allows Flash to reproduce all of these effects.
The Thing
The Thing is one of the toughest fighters in the Marvel Universe, used to duking it out with heavyweights such as the Hulk. The Thing's ultra-dense tissues and his rocklike skin make him a powerhouse even among Marvel heroes, and it seems hard for DC's premier speedster to equal such close-quarter effectiveness. After all, the Flash does not possess super-strength (even if Barry Allen's son does), but that doesn't mean he can't reproduce the same effects.
As first explained in JLA #3, by Grant Morrison and Howard Porter, the Flash's absurd speed can actually make him punch as hard as Superman, if not even harder. Mass and speed are closely related concepts in physics. According to the formula that force equals mass times acceleration, increasing the latter will also increase mass. Moving at close to lightspeed, the Flash's mass increases towards infinity, and in JLA #3 he puts it all into one "infinite mass" punch that sends a White Martian, a Superman-level threat, flying away to another continent. Flash lacks the Thing's rock-like skin, but due to his body being surrounded by his protective "Speed Force aura," his durability is likewise heightened to immense levels.
The Human Torch
Johnny Storm is the youngest member of the Fantastic Four, and he was a hothead long before the Cosmic Rays gave him his powers. The Human Torch's power is the mental ability to control heat energy and generate plasma-like flame. He can manipulate these "flames" for a large number of effects. Johhny can also fly because his plasma has a high hydrogen content, creating a cloud that can propel him throughout the air.
While Flash is certainly not able to reproduce all the effects of Johnny's plasma, he can at least make his own body "burn." Wally West once vibrated his hands at such speed that they started burning red hot, with enough heat to damage a powerful alien. More surprisingly, the Flash has been able to fly (or at least reproduce a similar effect) many times, using a variety of weird but inventive techniques, such as running on the water vapor of clouds or creating strong winds with his arms.
The Invisible Woman
Susan Storm is usually recognized as the strongest member of the Fantastic Four. In the beginning - back when she was the Invisible Girl - she was only able to turn invisible, but as time passed and her character developed, her powers also grew exponentially. The Invisible Woman has two main powers, based on the manipulation of ambient cosmic energy. The first is to make herself and/or other people or objects invisible. The second is to create psionic force fields which she can manipulate into a number of relatively simple forms. The size, number, and movements of these objects are limited only by her powers of concentration, and her mental energy directly affects the resilience of her constructs.
The Flash is actually able to reproduce both of Sue's main abilities. While he's able to run fast enough that he's not visible to the naked eye, vibrating his molecules to the right degree means the Flash can also render himself invisible even while standing "still." During Grant Morrison's run on the character in the late 1990s, Wally West discovered that if he concentrated, his Speed Force energy could be used to create solid constructs, which he used to compose his Flash costume. Later, he was also able to construct a vibrational prison of Speed Force energy to hold enemies. These constructs might not be as powerful or versatile as the Invisible Woman's, but then again Flash hasn't spent much time honing this aspect of his powers.
Mr. Fantastic
While he of course has his own superpowers, Mister Fantastic's main asset is his genius intelligence, which helps him and his team so much more than his stretching. Reed is one of the most prominent thinkers and inventors in the galaxy, and while Flash doesn't have his innate genius, he is a scientist (Barry) or engineer (Wally) and can use his "speed reading" power to research a huge amount of knowledge on any given subject, allowing him to become a world-class expert in seconds. Given Flash can also act on that knowledge faster than anyone alive - reconstructing buildings or creating unique inventions in moments - it's highly effective in all the ways having a genius on the team is.
However, can Flash actually stretch his body? The answer is no, but he can do something better. The Flash's ludicrous speed means he can literally be in two places at once (as proved in Flash vol. 2 #215, by Geoff Johns, Howard Porter, John Livesay, James Sinclair, and Pat Brosseau.) Fittingly, this comparison is the biggest stretch, but like Reed, the Flash can retrieve an object or strike an enemy while technically still on the other side of the room. Impulse, the future Flash, has also shown he can use the Speed Force to extend his Speed Force energy across time and space, creating energy duplicates of his body that can be in multiple places and time periods at once. Flash can't elongate his arms, but using this overpowered ability he can "extend himself" across space and time to do anything Reed Richards could (and faster.)
While his capacity to run at impossible speeds was Flash's original, and still most famous, power, writers have long expressed their creativity by giving the character the most bizarre applications for his super-speed. The Speed Force made Flash essentially capable of bending the laws of physics in ways that not even other superheroes can, meaning that DC's Fastest Man Alive can easily reproduce the entire power set of the Fantastic Four.