2-PAM or 4-PAM if I want to send data as fast as possible?
4-PAM transfers 2 bits ($log_2(4) = 2$) per symbol, while 2-PAM transfers 1 bit per symbol ($log_2(2) = 1$), so 4-PAM definitely transfers data faster.  The catch is that you need higher SNR to get the same bit-error rate (BER) that you would get with 2-PAM.