Hi @hsidney 

SharePoint REST API

You can use the SharePoint REST API to get the ItemCount for a list (per list)

/_api/web/lists/getbytitle('<list title>')/ItemCount

You can get the entire lists details using this SharePoint REST API (per site)

/_api/web/lists/

Warning: Result returned is a large amount of text that is not human readable

Cheers

If you dont have a lot of lists on the site you could run a workflow like this which query's each list.
Or even run 15 branches in parallel at the same time.
You could do one call web service call to get all the list ID's as above and then put the below workflow in a loop that goes through the lists collection variable.

Hi @hsidney 

SharePoint REST API

Here is a better solution using the SharePoint REST API that will return when Items is "gt 4500" 

/_api/web/lists?$select=Id,Title,ItemCount&$filter= ItemCount gt 4500

You can use Notepad++ or any online XML Formatter to format the XML results.

How to format or prettify XML in Notepad++ | Code2care

Google "SharePoint 2013 REST API" for resources.

Getting Started

Open your SP site in browser

https://{site_url}/

Just paste this after your Site URL

/_api/web/lists?$select=Id,Title,ItemCount&$filter= ItemCount gt 4500

The Final URL

https://{site_url}/_api/web/lists?$select=Id,Title,ItemCount&$filter= ItemCount gt 4500

Cheers