Per accident I committed twice because I forgot to add two files. Can I remove a specific commit from the log?
Why? Let Subversion keep the history - that's what it's for. Check in early and often. There's no need to erase commit #4.
The simple answer is "no", because Subversion doesn't know how to resolve the case when you add a commit, someone else updates their checkout, and then you remove the commit from history. There might or might not be a complex answer involving surgery on the Subversion storage.
In your case, it's just necessary to modify the commit comment associated with commit #4 to reflect the actual changes you made. You can do that with Subversion by modifying the SVN repository configuration. See the Subversion FAQ.
If you can delete current repo and create new with new history, you can try:
svnadmin dump
for getting human-readable (barely, I have to say)
repository-dump, grok format and edit dump, delete repo, svnadmin load
to recreate the repo