I also would have little doubt the -3 would handle the pressure, however as you know, you will amplify the pressure in the tubing when you downsize. I would lay odds that you would blow seals before you would burst the tubing, but why not do the math? Grab a standards book, look up the specs of the tubing in the alloy and temper you are considering, and go at it. There are even handy calculators on the web that you can plug the figures into if you know them. You don't even need to come up with the actual PSI, just a factor between the -3 and -4. That will give you the difference in a percentage of safety factor.