Expandable cart with unfoldable rear frame

An expandable cart with an unfoldable rear frame, comprising a first column, a second column, a third column, a fourth column, a first folding mechanism, a second folding mechanism, a third folding mechanism, a rear frame mechanism, and a bottom support mechanism, wherein

• the first column, the second column, the third column, and the fourth column are arranged at intervals in a rectangular shape, and have lower ends all connected to the bottom support mechanism, and the bottom support mechanism is allowed to be folded upward;
• the first folding mechanism is connected between the first column and the second column, the second folding mechanism is connected between the third column and the fourth column, and the third folding mechanism is connected between the fourth column and the first column;
• and
• the rear frame mechanism is disposed between the second column and the third column, a lower end of the rear frame mechanism is rotatably connected to the lower end of the second column and the lower end of the third column at a first limited angle, and an upper end of the rear frame mechanism is detachably connected to an upper end of the second column and an upper end of the third column.