Elle here from Shopify's Social Care team. Hope you're having a great Tuesday!
To change the number of products in a row on a collection page, you'll need to edit one line of code within the collection.liquid template. Head to Online store > Themes > Actions > Edit Code > Collection.liquid (under Templates).
Search for the following line:
<div class="grid__item small--one-half medium-up--one-fifth">
Change 'fifth' to 'quarter' so it looks like this:
<div class="grid__item small--one-half medium-up--one-quarter">
Once you save this change, you'll see 4 products in a row on all of your collection pages.
Hope this helps! ?