How can I find and specify the address of my adafruit MMA8451 accelerometer in arduino code?
You can't have 4 of those on one bus. The maximum is 2.

The pin SA0 defines the least significant bit of the I2C address. From the datasheet:


  7 | SA0 | I2C least significant bit of the device I2C address, I2C 7-bit address = 0x1C (SA0 = 0), 0x1D (SA0 = 1). 


In fact, as it states on the Adafruit website (if you'd care to read it sometime):


  A is the I2C Address select pin. By default this is pulled down to ground with a 10K resistor, for an I2C address of 0x1C. You can also connect it to the 3Vo pin for an address of 0x1D


If you want more than 2 you will either have to have multiple I2C busses or use an I2C multiplexer (such as this one) to select different branches on the same bus.