I am not sure if you mean you want to set a maximum quantity per order or maximum order sum. There is only a slight change to set up each. I am assuming you mean the maximum quantity. Here are the options you need to configure in the plugin’s settings:
General > Amount limits > Maximum General > Amount types > Quantity Products > Per product Category > Enable
Then go to the category whose limit you want to set as 4 items and edit it. You will find a new option for maximum quantity, set it to 4.
Now a customer will only be able to order a maximum of 4 products from this category.