Two options
I have used a rachio 3 controller for years and I am adding to my lawn. I require 4 more zones. is there a way to add another controller and not have it activate 2 zones at once?
I have 6 zones of shrub and flower with the control valves not near each other). could I run them on 2 zones ( with an external 24VAC relay (2) ) which would free up 4 zones to run the other new Lawn area?
At this time, no. You will have to manually set schedules to keep them from overlapping if water pressure is an issue for you.
I have two controllers on my property, and while I try to limit the overlap of zones, I have little to no issues if 2 zones kick on at once…
As @tmcgahey mentioned, setting up the schedules so they do not overlap is certainly an option and having that automatically occur is not available at this time (others have requested setting up two controller to act as one virtual controller). Also, if they do overlap, you may or may not have issues with pressure (the common wires should be separate for each controller without doing some other things).
The second option that you pointed out should work (again, depending on pressure, etc.). The relay that should use low enough amps for the Rachio side and support enough amps to drive the combined valves (of course the power source too).
Depending on whether you have a master valve or a pump relay that needs to be powered, Rachio can fire off 3 zones without issue. If you don’t have a MV or pump, then wiring 3 zones to function as one will work to cut the number of zones down. That is actually what I did with my grass zones. I switched from fixed spray nozzles to Hunter MP nozzles, which allowed me to consolidate the number of grass zones I have. Someday, I’ll re-pipe the zones together the “right” way, but for now, wiring them together nets me the same result.
That is probably true with more modern valve, but I personally would double the amp draw of each valve.
I suppose there are some really old solenoid valves out there, and you might be right. But anything within the last 20 years or better would have similar specs, so define “modern”. 
• 24 VAC solenoid
Yes, “modern” is a bit ambiguous. I was just thinking of changing to “old” or “new”, but those are probably a bit ambiguous too. 