Unfortunately I don’t think it’s possible at the moment. You could set minimum quantity to 2, step to 2 and maximum quantity to 4, but that would make quantity of 1 invalid. I could try to add this to the plugin, but I’m not sure how would admin set allowed quantities… I mean would listing all valid quantities as comma separated text (I mean by admin) would be good enough, e.g.: 1,2,4? And do you need this on per product basis (i.e. different for each product) or these quantities would be applied to all your shop’s products?
We’ve just released new plugin version 1.5.0. After you update, you will find new “Exact Quantity” options section (in “WooCommerce > Settings > Product Quantity”). This should do exactly what you’ve described, i.e. set allowed or disallowed quantities as a comma-separated list, e.g. 3,7,11.
Please let me know if something is not working as expected.