A 6wt is your best all-round rod. You can catch pinks on it, and coho, and it's still alright for most trout fishing. That said, if you're targeting local lakes for rainbows, those are some small fish. A 5wt would be more appropriate for those, but then you're borderline undergunned for the pinks.
A 4wt is definitely too light, as others have said. Can you still LAND pinks on a 4wt? Of course! Should you? My answer is an emphatic "no".