Hi,
The problem is that, with some specific browser width, the products block content are too large. So the content is larger than it should be, taking more place than the 22% of the page width.
It could be the add to cart button or maybe something else. It require tests to determine it.
I just test to remove the margin between blocks (which is a margin of 2%) and it display the four blocks good. But it's not the right solution, just a test.
I think you have to add some specific rules with the small width (using the @media) in order to change the design of some elements for these small screen sizes.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.