Hi,
Well, you might end up with an issue with any upgrade based on what you use and what you customized and what other extensions you have on your website.
So it's impossible for me to guarantee you that you won't have any issue with an update, be it HikaShop or anything else.
It's always better to first test the update on a copy of the website before doing the update on the real website.
That being said, HikaShop 4 doesn't have huge structural changes like HikaShop 3 had over HikaShop 2 (whole new cart and checkout systems). While HikaShop 4 has many nice things over HikaShop 3, the changes are a lot less critical and so the update has a high chance that nothing will go awry.
