Hi @gm1967 , welcome to FUD!
Dexcom may have some limitations on which phones can use Follow due to the FDA. The S4 is a pretty old phone.
You can use xDrip+ successfully to make a Security Group among your Android phones of any age.
What you need to do is set the PWD (person with diabetes) up with xDrip+. Then load xDrip+ on all the other Android phones you want in the group. Then copy the settings from the PWD’s to the other Androids. (Do this through the xDrip+ app). Then set up the Security Group on the PWD phone.
The PWD will need to keep his phone within his proximity with a Wi-Fi or data connection to maintain a connection with the rest of the group.
If you trust the Dexcom algorithm more than xDrip+, just run the PWD phone in native mode.
Also, on your daughter’s phone, set up sync with Dexcom Follow in xDrip+ and your wife can continue to use Follow on her iPhone.
Any questions, let me know. Good luck!