I need someone to customise Shopify in order to add a 4th option to variants (not using a plugin)

We have a product which has 4 options, we are currently using a plugin but it slows the website down and does not work well with our Inventory Management system. We are looking for someone to code directly into the shopify platform to enable a 4th option. Please see attached.

PLEASE DO NOT CONTACT AND RECOMMEND A PLUGIN, WE ALREADY HAVE THAT SOLUTION.