The question covered in this guide only arises if multiple users are using Steam on the PC you are using. Otherwise, there will always be only one ID folder in the userdata subfolder of Steam.

To find Your personal SteamID we need the assistance of a converter, and your profile link. While there are multiple converters available, I like to recommend the one by SteamDB.

https://steamdb.info/calculator/

Copy the URL / link of your Steam profile into the converter and you get details about your SteamID. The number of interest for you to find your save game location is the last number of Steam3 ID.

Steam3 ID: (Letter:Number:Number)

Your Save Game Location
Local: ..\Steam\userdata\YOURID\1669000\remote

with Steam's default installation setting it can look like

C:\Program Files (x86)\Steam\userdata\YOURID\1669000\remote

Replace YOURID with the last number of Steam3 ID and you have identified your exact save game location for Age of Wonders 4.