Im generating debug logs from both machines. The app on the 4 GB machine doesn't crash so much as it just freezes solid, and I mean solid. Repeatably. But I don't think it's a RAM issue, as only about half the RAM is even in use. Very low "memory pressure".

I'm running the other Mac now, syncing up the block chain from scratch with a fresh wallet. This will take a few days, I'll return with more into once this is done.