Bottom Line Up Front (BLUF): Suno V4 is the base model of the fourth generation. Simple and predictable, it is a good introduction to music generation: a short description and you get a finished song with vocals.
V4 is handy for quick tries and small tasks that do not need maximum audio detail. It makes popular-genre songs reliably and is great for testing the idea of a track itself.
A simple description mode is available; for your own lyrics and settings choose the newer versions — V4.5, V5 or V6. The result downloads right away.
Yes, it is usually the most affordable model of the section; the exact price is shown before you start.
For a first try and simple tracks — yes; for quality prefer V5/V6.