The amount of power RAM consumes is only a small fraction of the total power consumed by the entire device. If you intend to make heavy use of the device, play games and multitask a lot then 8GB of RAM might actually slightly increase your battery life since your device needs to swap less (moving data from/to RAM to storage if the RAM is full), and swapping costs you some battery life.
Smartphones have been around for over a decade now and I am yet to encounter a phone that died to heavy rain, even before IP ratings became common. The FP4 has an IP54 rating, so should be fine even in heavier rain 
https://en.wikipedia.org/wiki/IP_Code#Second_digit:_Liquid_ingress_protection