As the pp said, if all is fine, you may not need to worry. But if there are things going on that don't seem right, then OALD/oversupply can be the culprit.
You said you are using 4 hour blocks of time and BM's are still pure liquid and bright green. While some variances in color are normal and some babies can have green BM's and be fine, if you are feeling that ds is having a hard time, then the BM's would be a way you can guage how things are going.
Rarely, but sometimes, mothers in your situation have to go to even longer blocks of time: 5-6 hours, to downgrade supply. You might consider this.
And you are right in that some babies just aren't bothered by this, while others are. DS is lucky to have such a caring and concerned mom!