I'd just try it on your current OS, if it does not work consider upgrading to 4.5; I think plazmic only exports to it because the recent changes from Plazmic 4.5 onwards may only be supported by the newer OS's.
You could put on the page with the theme "Theme designed for 4.5; Try it on a lower OS at your OWN Risk. I do not accept ANY responsibility if you brick your device."
I don't really think the export OS will make too much difference, the most I can see happening from a theme designer/developer's point of view is some compatibility issues e.g animations not working; just staying static.