i'm so worried about 3.0 sprites getting corrupted in 4.0!

There is no Scratch 4.0 though.

As always, I'm sure the Scratch Team's goal is to ensure that Scratch 3.0 projects work in Scratch 4.0 when it eventually comes out.

4.0 isn't confirmed to have begun, exist or been planned to release though….
But if I take a guess i'm sure your sprites will still work in 4.0

Edit: just read the posts above

Considering that you can upload a project made in the 1.4 offline editor in 3.0 and it will (mostly) work fine, and considering that the .sb file format is incredibly different from the .sb3 format (.sb2, the 2.0 format, is more similar to .sb3, having the same overall file structure), the Scratch Team is likely to keep up backwards compatibility with older versions of Scratch- especially the version of Scratch where the most Scratch projects in history were made.

Whenever Scratch 4.0 comes out, I am certain that it will remain compatible with content created in past versions.
Scratch 3.0 is backwards compatible all the way back to Scratch 1.0, and even older versions to some extent.

However, if Scratch 4.0 comes with its own redesigned editor, then the existing 3.0 editor will probably not remain available on the Scratch website.
It may remain available for download, as is the case with Scratch 1.4 and 2.0, but probably won't receive any further updates or support.

If you are inquiring about the editor's appearance, then it is likely a yes.

The most closest thing as scratch 4.0 is turboWarp or scratch lab