Show 4 category posts rather than 3

I tried to follow the details posted in this thread
https://wordpress.org/support/topic/home-page-customize-post-grid?replies=4
however the posts page is not my home page

I am showing the posts via category rather than a page and I have tried amending the category page but to no avail.

Any help would be appreciated