Why does the sound play faster
Let Sic be the i-th sample of the c-th channel, you started with stereo (2-channel) 16kHz. Your audio was like this
 s11, s12, s21, s22, s31, s32, s41, s42, s51, s52
When you made it a 4-channel audio you reassigned the same infromation to different channels like this
 s11, s12, s13, s14, s21, s22, s23, s24
In the stereo configuration you were using 2 samples every sampling period, so at 16kHz you play 32000 samples per second. With four channel you use 4 samples every sampling period, if you want to play 32000 samples per second your sampling rate needs to be 32000/4, 8kHz.