As your I485 has been pending for more than 180 days, you are eligible for AC21 protections.
Please read the EAD/AP thread for things you need to consider to keep both status'es valid. Look at the top two threads.
We also had a
discussion on switching between EAD/AP and H1B. See that thread for
all the law points. Instead of reinventing the wheel, I will give short answer.
1. H1B will be void if you don't manitain H1B employment.
2. If H1B is not valid, then H4 is invlid. Hence, H4 EAD becomes invalid. It may take some time for the system to update. However, don't work on the H4 EAD. You will face consequences later on. The law is very strict.
Bottom line is browse that thread and come up with your own understanding. You can setup some time with attorneys to ask this question and double check the interpretation. Good Luck.