I am testing my extensions on 4.0.0-beta1-dev and found that the sample data plugin is not visible in the sample data module. I do not have such an issue on the latest Joomla 3 installation.
Did a check that the code of onSampleDataGetOverview() method of my plugin and the blog sample data plugin is the same. So I am confused where the issue is.
You can see that the plugin is enabled:

But nothing in the sample data module:

Here is the screen of my J3 installation where everything working fine:

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Omg :) True it is case sensitive. Now it is working. Thank you.
| Status | New | ⇒ | Closed |
| Closed_Date | 0000-00-00 00:00:00 | ⇒ | 2020-01-26 20:24:45 |
| Closed_By | ⇒ | b2z |
Looks like plugin event names are now case sensitive. In J3
onSampledataGetOverview,onSampleDataGetOverviewand any other casing work but in J4 it has to beonSampledataGetOverview.