The 4PAM transmission scheme employs the alphabets {+/-1, +/-3}.
The generated random alphabets are passed through an additive white gaussian noise and the symbol error rate is computed.
The computed symbol error rate from simulations agree with the theoretical derivation of symbol error rate.
For description of the theoretical derivation of the symbol error rate, kindly refer:
http://www.dsplog.com/2007/10/07/symbol-error-rate-for-pam/