Would it be more efficient to use two 2-ton systems or should I use one 4-ton system with two zones for a home air conditioning system ?

I'd go with a single unit. What you MIGHT gain in operational cost and efficiency you would nullify by buying 2 systems. 2 completely separate systems is a lot more expensive than a control panel and zone valves. Panel is $125 and zone dampers are about the same. I have a 4 ton unit, Honeywell Control board and 3 zone controllers. 10 years old and never an issue.

How big is your house in square feet?

Hi ma0641, thanks for your answer, ended up considering 1 !/2 3 1/2 or 5 ton with 3 zones. Went with the 5 ton with 3 zones, appreciate the help. Incidentally, lost my password for the site ----a menu says that ASK will send a link to my email but does not. Hence user name now Freddylee1 .

Hi talaniman, I've semi-committed to one 5 ton with 3 zones. Had password problem with the site, Have 3800 ft, 9 ft ceilings with small amount of cathedral , would like your opinion also.

A lot depends on the solar exposure, where you live, windows etc. Did you do a Manual J to determine load? Manual-J Heating and Cooling Load Calculations
Manual J (http://homeenergypartners.com/pages/menu-links/manual-j.php) is the name for a specific protocol (often called “Heat Load Calculation (http://homeenergypartners.com/pages/menu-links/manual-j.php)” or “Cooling Load Calculation (http://homeenergypartners.com/pages/menu-links/manual-j.php)“) used to determine how much heating/cooling a home needs to stay cool and dry in the summer and warm in the winter. This load calculation process was developed by engineers in the heating and air conditioning industry and has been used for decades to accurately size heating and air-conditioning equipment. After completing this load calculation process, one can choose a properly sized piece of machinery to satisfy the load.

It starts with knowing the area you are heating and cooling, and the particular region you are in.

https://asm-air.com/airconditioning/what-size-central-air-conditioner-for-my-house/

In addition,

https://www.proudgreenhome.com/news/can-you-do-a-manual-j-hvac-calculation-in-less-than-60-seconds/

It will lead you here,

https://www.youtube.com/watch?v=VHoyXTNCH9s

What MA064 explained is spot on, you have to do the math.