Hi Rajan,
Welcome to the forum!!
Just so we’re doubly sure that we understand your project - You would like a system in place where you are able to play 4 different tracks on the same audio line at the same time? In every project there will be multiple ways to build it, other members of the community might have a faster way to do things but here’s my idea:
If you only need to be able to select one or two to be playing at once I would recommend check out this topic where Jim, Ross and Oliver take a look at the DFRobot Mini player
To make it quickly definitely grab something in stock at the moment, at the time of writing there are 6 of the DFRobot players in local stock, 3x Sparkfun Audio sheilds, NO MP3 Triggers and various Adafruit Audio triggers (Uses .WAV files, they come with and without amp’s and varying amounts of memory).
My goto dev/controller board at the moment has been the Raspberry Pi Pico, it allows you to use MicroPython which decreases development time significantly
You can check the stock of an item by typing in how many you would like into the product page, the text above will update with a live stock count 
If you need a hand with anything else let us know!
Liam.
