Running the live image on the MacBook Air 11" 2011 (I believe) works fine.
The graphical installer collects all information (such as region, keyboard, partitioning and so on). When I click ‘install’ it waits for several seconds and simply quits without any kind of message. Nothing is installed. The computer remains as it was before.
The online manual says to run sudo setup in a terminal. Setup is an unknown command.
How do I find out what’s amiss? How can I install Manjaro on that MacBook?
First of all, any searches for answers will need the correct identifier, in your case MacBookAir4,1.
Secondly, your issues are most likely due to the low amount of ram of that model. The 2GB version probably won’t install/run at all and a distro like antiX would be a better fit. The 4GB one might run ok, in any case I’d download the Xfce minimal version.
Have a look at @linux-aarhus suggestion of using manjaro-architect to install the system, all you need is a working internet, architect will lead you through the process and pull the required package according to your choices during install. I recon that once that’s done you’ll have a fairly usable machine since all hardware is supported and your drive is fast.