Does A Higher Mah Battery Last Longer. Wiring two 4000 mAh batteries in parallel gives you double the capacity yes. The battery for the Focus 1250 mAh died so I tried to use the 450 mAh one.

There are several ways to define battery life. Of course this also means longer battery life for a given usage. This is its Continuous Rating or C-Rating.
This refers to the amount of power a battery can deliver.
So batteries with higher mAHs are more desirable because they can last for a longer period of time since they have a higher current capacity. You will be surprised how your batteries will last longer. So that is definitely an option as well. Its a tough slog to create batteries that last longer because battery.
