Should be fixed in 4.0.1.

yes, this bug is very bad granted it is still beta.
I'm eagerly looking forward to get new version!!

I've got the same problem, but it seems to be fixed in a not-released version:
https://winscp.net/eng/docs/history

• Bug fix: For automatic transfers (uploads from editor, synchronization), the transfer was incorrectly automatically “resumed” when source file was larger than destination one, leaving content of file corrupted (FTP only). The issue may also make uploads from editor impossible in case server does not support (text file) transfer resuming.
  

So lets hope the new version gets released soon :)

I can confirm this. It's really disappointing because I liked this feature a lot and was excited when I was able to use this with FTP only servers I developed on. The bug garbled quite a few of my files before I figured out what was going on. :(

open remote file for "Edit" ( Internal or external editor)

If using "Open" command then not this bug.

This is bug (ONLY FTP MODE):

Content test file :
----------
begin
my text
end.
---------

1) open remote file for "Edit" ( Internal or external editor)
2) add text before end of file
---------
begin
my text
addtext
end.
---------
3) save file (not close) ==> file will be sending to remote site
4) closing editor
Now see remote file content :
---------
begin
my text
end.ext
end.
---------