Re: 4x weak - to battle or to switch?
Leave it in.
My Pokemon seem to have luck, and with Aggron now having Sturdy being more useful, I don't have to worry it being one hit KO.
With others, they are generally strong enough to take it out or leave a large dent. The CPU is stupid though and rarely does something threatening...Although B/W DID improve this, my strategy is the same.