Carbs? Easy. You just mess with screws til it runs right, right?
Altitude adjustments require changing the jets (or metering rods, as is the case on my car). Then you break out the quad vacuum gauge and mess with screws to tune/synch them.
But with a ~3,000 feet difference, you're usually able to split the difference and it'll run nicely (/nice enough) in both places. It seems to be the consensus that a motor tuned for sea level won't see much issue until 5-6,000 feet. It'll be a bit down on power at 4,000 feet, but that's pretty much unavoidable unless you turbocharge it.