You don't need a finished song to use Suno well. A four-chord loop or a phone recording of you humming is plenty to start from.
Start with what you already have
Record your idea exactly as it is, even if it's just you humming a melody or playing one chord progression on a loop.
Describe that recording back to Suno in your style prompt as closely as you can: the instrument, the feel, the tempo you played it at.
Don't worry about it being incomplete. A verse-length idea is enough for Suno to build a full structure around.
Build out the structure
Use structure tags like [Verse], [Chorus], and [Bridge] in the lyrics field to control where your idea shows up in the arrangement.
Generate a few versions with different genre tags on the same core idea. The chord progression stays recognizable even as the style shifts around it.
Pick the version that keeps the most of your original feel, then treat that as your new starting point.
Keep your style consistent as you extend
If you extend the song past its first section, paste your original style tags back in every time.
Skipping this is the most common reason a song sounds like a different track halfway through.
Keep your BPM in every extension too. Tempo drift is just as noticeable as a genre change.
The song was never really missing from your four chords. It just needed the parts you hadn't gotten to yet.