First, congrats on making it to 4 mths!!!!
Everything about breastfeeding is supply/demand and every woman's body is different.
So, your body will regulate itself based on how much pumping and feeding you are doing. If you want to build up a large stash then pump until it is empty. If you are happy with what you are doing then keep going as you are. Just know that later, if you want to pump more you might not be able to because your body will have adjusted to your and your lo's feeding/pumping schedule (well, you would be able to, but you would have to work at it for a while to up the supply again).
9ozs from one breast - yep, I use to do that, but ds doesn't eat as much now so my body has adjusted.
Two breasts vs. one breast - this is where every woman's body is different. Some start off with double breast feedings from the get go, some transition to this later and some never transition to this. For me, I only fed off one side until about 2 months ago. At that time I went on an antibiotic that caused me to have supply issues and I had to go to two sided feedings (now, I sometimes do one and sometimes do two but I always offer both). I would recommend feeding from one side until it is empty and if, at that time, your lo ever appears to still be hungry then switch sides. If you are ever unsure, then offer the other breast - remember, you cannot overfeed a breastfed baby. I will say that my body went through a transition around the 6mth mark where it started to regulate itself better. It is around this time that the milk gets thicker in texture too so you will notice some changes in supply around this time.