When you see a title like "Song Title (Series Name OST Part.4)", the label in the parentheses is doing two jobs. OST stands for original soundtrack, which means music made for a drama, a film, a web series or a similar production. The words before OST are the name of that production, so the parentheses tell you where the song belongs. If you already know the series, you know the home of the song. If you do not, the label gives you a name to search before you press play.
The Part number is the piece people most often misread. A production usually releases its soundtrack songs one at a time over its run rather than all at once, so the numbers climb as new entries arrive. Part.4 means this song is the fourth entry in that production's soundtrack release run. It is a counting label, like an episode number. It does not rank the song, it does not say how big the song is, and it does not mean the song is better or worse than Part.1 or Part.2.
The singer credited on the release is the performer of that specific entry, and different parts commonly have different singers. That is normal for soundtracks, so do not assume the same voice carries across every Part of one production. If you follow a singer and spot their name next to an OST label, you are looking at one entry in a longer run, not necessarily the whole soundtrack. Also, you can stream the song on its own. Watching the production is not required to listen to the music, though some listeners like having the context.
To confirm details, start with the title itself and read the parentheses carefully. Then open the description, where credits usually live. Check which channel uploaded the video, since that tells you who is presenting the release. Finally, look at the production's own announcements, which are the best place to see how its soundtrack is being released. A few limits are worth knowing. Formats vary, so you may see Part.4, Pt. 4 or Part 4 for the same idea. A title does not tell you the story or what the song is about. And a title alone does not say whether more parts will follow.