You said that Google reserves 4 IP address in a subnet. Normally in Networking with CIDR we reserve the first IP for the default gateway and the last IP for broadcast. What are the other 2 reserved for by Google and what are they?
You are correct when saying that the first and last addresses are reserved as per normal networking practice, but Google (as well as AWS for that matter) also reserve 2 extra addresses – the second usable address in the range is reserved for the default gateway, and the second last useable address is "reserved by GCP for potential future use".
This link has a bit more info as well as a really useful table which spells out the reserved IPs and what they are used for:
(Just scroll down to the "Reserved IPs" section!)