Mk4 will use Aria with 3xAA Lithium which have 3000mAh http://www.farnell.com/datasheets/9583.pdf
I’d say the project is definitely one that you want to pop out dead cells and replace. You can’t have downtime for charging.
Yep to the pop out a must. My Nest protect came with 6 AA Lithiums. Also there restrictions on the carriage of Lipos via air transport and in the UK we can’t put them in the post they have to go via UPS etc.
btw, there is a commerical imp-powered motion sensor (and door sensor) available; it was designed a while back so uses the imp card vs a module. The motion sensor drain is very low - around 10uA.