CSS to decrease size of thumbnails in product gallery

I wonder if anyone can help me – I have spent hours trying to find the CSS to decrease the size of my thumbnails in the product gallery with the intent of having 4 pictures in a row.

I have managed to change the number of columns in a row to four, but then i seem to have a row of three with one underneath. I need to decrease the size of the thumbnails to fit 4 to the width of the page but i don’t know how.

Frankly the images are way too big at the moment and they are making my page look ugly. I would be so grateful if someone can help me!

https://allthatglittersweddingstationery.co.uk/product-category/wedding-stationery/