Rear brake pad wear

It is unlikely that you installed undersized pads but it could be the pads are just poor quality.

I am attaching the process below on how to replace the pads so you can review just to make sure you performed the repair properly.

However, before you go through any major processes, how much have the pads worn? Did you measure them? How long as it been since you installed them?

The rear pads are down to 4MM since June 2020 and between Aug 1st, 2020 until March 2021 I only drove it a few time a week because I was traveling for business. I may have put 10,000 miles on the car until today. It's probably the crappy pads I bought. What specs should I look at to know if I'm buying good pads?

It is hard to put a number of what you "should" be getting as there are too many variables including, pad quality and driving habits.

However, 4mm in 10,000 miles points more towards the quality of the pad as you indicate more than an issue.

Especially if you put ceramic pads on as they are soft and wear quickly. However, they are quiet and have very little brake dust.

If it were me, I would keep an eye on it and when they are low, replace them with a different set and retest to see if they wear just as quickly. I would go with a semi metallic and see what happens.