That will be different for many people. As annoying as it is I know guys who have pretty visible abs, yet the aren't super lean in other areas, while for some they can be pretty shredded and just only beginning to see them.
In general I'd say at least 12% but usually around 10-12 (for what I'd consider really still being able to see them, not what I see a lot of people try to pass off as actually seeing them).
In the end though I wouldn't gravitate or focus on a number. Each person is different and my best suggestion is to just keep going until you get there yourself and then have a better understanding what it takes and where you need to be to get there. If you really can see the top 4 you may be getting close so keep at it!