Not knowing the URL of your website, I can't say much.
The "Force SSL on checkout" setting of the HikaShop will do only one thing:
When you access the checkout of HikaShop, it will check if you access it with https or http. If you access it with http, it will redirect to https.
This supposes that the SSL certificate on your web server is already setup properly. To check that, just open any page of your website and add the "s" after http. If the page don't work, the SSL certificate is not setup properly and you need to check that with your server administrator/hosting company.
If your SSL certificate is properly setup and that you still have a loop when you turn on that option, it means that something else on your website is redirecting https to http. It could well be a system plugin. Disabling them one by one should help you find the culprit if it comes from one of them.
Then regarding that you have 404 errors when you access HikaShop, without looking a the URLs, it's hard to say. Often it comes from the canonical URLs you entered in your products/categories which are not valid anymore. In such cases, turning off the "force canonical URLs on listings" setting of the HikaShop configuration helps.