During the winter between his three-year-old and four-year-old season, Assault underwent some changes. He matured a lot, gaining good looks that he lacked as a three-year-old, and his personality made a positive change that would make him more vicious on the track. However, the change negatively affected the time he spent at his stable. He was constantly hungry, impatiently throwing tantrums and attacking handlers if he did not get his food on time.
Assault, dubbed the Club-footed Comet because of his injury as a young horse, began the season 1947 by winning Grey Lag Handicap, with jockey Warren Mehrtens in the saddle. Eddie Arcaro rode him six days later in the Dixie Handicap. Assault then defeated Natchez and his old rival, Stymie, by two lengths in the Suburban Handicap.
After that, he defeated Stymie in the Brooklyn Handicap with 133 pounds on his back. With that victory, he surpassed Whirlaway's earnings and became the world's leading money winner. He came close to winning America's unofficial Triple Crown, the Suburban Handicap, the Brooklyn Handicap, and the Metropolitan Mile, but Stymie ended up winning the Metropolitan Mile.
Next, Assault entered the Butler Handicap, along with Stymie and the great mare Gallorette. On the homestretch, he was boxed in behind the two other champions. Incredibly, he managed to squeeze into an impossibly small opening, beating Stymie by a head.
Assault's seven race winning streak was put to an end when he came third to Stymie in the International Gold Cup at Belmont. Even so, the third place prize was enough to keep his status as leading money winner.
Then a match race with a purse of $100,000 was proposed between Assault and Calumet Farm's top gelding, Armed. When Assault was injured during a race, the match race was postponed from the original date of August 30 to September 27. Assault pulled up lame on the September 22. Many people, including Eddie Arcaro, strongly believed that Assault shouldn't race. In spite of that, Robert Kleberg decided to race his horse since the race was for charity anyway. Assault was much too sore to come close to winning, and lost by eight lengths.
After that, Assault went to rest in South Carolina. With him out of the way, Armed earned 1947 Horse of the Year honors and Stymie became the new top money winner.
A few months later, Assault was back on the track, but not in winning condition. After coming fifth in the Widener Handicap, he rested another few months. In August he tried once more to win. However, he came second by a nose in the Brooklyn Handicap. In the Massachusetts Mile, Assault's performance was even worse, and he came fourth as well as developing a bleeding problem.
He won the Edgemere Handicap, yet it wasn't enough to compensate for his losses in the Manhattan and Grey Lag Handicaps after that, so he went into retirement in Texas.
Tests showed Assault to be sterile, so he was turned out with eight Quarter Horse mares. Surprisingly, he had two sons and two daughters in the spring of 1951. Also, he raced three more times, coming first, second, and third, and earning $675,470 in his lifetime. Though they tried breeding him with thoroughbred mares, he got no more offspring. After passing away on September 2, 1971, he was buried on King Ranch, Texas, where he was born.