"The Sims 4 permits you to put your own music into the game and play it on the pre-existing radio stations, which are sorted by genre. Here's how to add the music to the game:
Choose which songs you want to put in the game. They must be all .mp3 files with a bit rate no larger than 320kbit/s.
Go to your game's "Custom Music" folder which is normally located in C:\Users\YOUR USERNAME\Documents\Electronic Arts\The Sims 4\Custom Music on Windows and go into the folder for any of the seven stations.
In your selected station's folder, paste (right click and select "Paste" or press CTRL+V) all the songs that you want there.
Start your game, turn on the stereo and select the appropriate station and see if your music works. Note that it will be shared with the audio shipped with the game for that station but those tracks can be disabled individually in the "Game Settings" menu. Note that in live mode, the stereo has three volumes: low, medium, and high. If you cannot hear your music on low setting, you may be able to hear it on medium or high setting or by navigating your Sim closer to the stereo. (High settings may result in unfriendly apartment neighbors.) Another way to test this is to start your game, go into game settings, Game Options, Music, then select the station, locate your music in the station and test it. "
Source:
Thesimswiki