How to convert to mp4 on the fly?

Question

glc-play [stream file] -o - -y NUM | mencoder -demuxer y4m - -nosound -ovc x264 -x264encopts qp=18:pass=1 -of avi -o video.avi

from http://nullkey.ath.cx/projects/glc/wiki/HowtoEncode

takes a glc file, dumps it, and encodes it into an avi on the fly.

Is there a way that I can modify mencoder to dump out *.mp4 instead of *.avi?

Thanks!