4 full bridge load-cells with the HX711
Four 3-wire load cells can be combined into one 4-wire bridge using the following schematics:


Here R#C and R#V represent constant and variable resistance inside each cell. W/B/R is one of the common color combinations for load cell wires, for +/-/C respectively. Your sensors can have different wires, of course. If that is the case, use multimeter to find the common wire. Sparkfun has convenient load sensor combinator board and very informative hookup guide for these. 

Unfortunately, Wii Fit board uses 4-wire load cells that cannot be combined as above. You have two options here, either replace the cells and connect them as shown, or connect all 4 in parallel and hope that the load will not be too much for the voltage regulator.

Some other, more complex, options are to connect to the balance board via Bluetooth or hack the internal PCB to tap into I2C line. See here for related discussion.