One piece of advice: figure out how good you'll be on an away by using your M3 performance as a guide.
What do I mean by that? I agree with the OP's main point: aways are worthless if you can't impress/fit in with/get along with the residents. People who are humble and hard working and have decent to good social skills tend to do the best. People who come off as arrogant, uninterested, or awkward tend to do the worst. Most people fall somewhere in the middle.
If you consistently impressed your home residents on your core M3 rotations, you have a good shot at impressing your away rotation residents - the core qualities that got you that performance will apply. But if you failed to stand out, or worse - were actively disliked by the residents - probably best to limit your aways.
It's a month long interview and not everyone interviews well. If you're in doubt, ask (honest) fellow classmates or ask your M3 rotation residents yourself.
(From a former resident's perspective - we really looked closely at our med student rotaters and tended to favor them as long as they impressed us. They didn't have to be AOA-stars academically, but we wanted kind, humble, hard-working potential residents. It amazed us how many students came off as disinterested or condescending. Most of those students probably didn't realize just how off-putting their personalities were, and would have been helped by getting some honest feedback earlier in their lives).
Also: another mistake M4s make is when selecting aways, they tend to focus only on where they want to go, and they don't spend enough time researching the program. E.g. "I go to school in Nevada but want to return to Boston for residency" --> well that program you just applied to might have a track record of not favoring away rotaters, or not offering IIs to a large number of away rotaters. If so, you just wasted a lot of time/money/effort.