4 months is not too late to get the ears standed, but it may get harder for him. First of all, keep them trim like they are now. You may want to tape them for a few days. I had to tape Bengi's ears and it worked. I followed these instructions:
http://members.cox.net/jrogan/ears.html
Now, remember that some yorkie ears are too heavy and might never stand, so don't worry if they don't stand, the floppy ear look is very cute, specially with larger yorkies
