Use the product filters to narrow down the returned data to only those you need. This way, you will also get the responses faster. Below is a request example with the part where filters go bolded:
https://api.api2cart.com/v1.0/product.list.xml?api_key=<your_api_key>&store_key=<your_store_key>&start=0&count=4¶ms= <filters that you need separated by commas>
In the params field, use any of the filters listed below. To get all the fields allowed by the shopping cart in one response, use force_all.
System filter params: