The amount of TBT you earn is based on the length of your post. So if you post several paragraphs, and then delete all except one, you'll lose TBT. If you write one sentence, and then add several paragraphs, you'll gain TBT. If you edit a post and change one word, you probably won't gain or lose any TBT, but you'll see in your currency log that you lost & gained that TBT back (recalculating, basically).
Check your transaction log (Currency tab --> Bells --> Transactions). It should say you lost those 4 TBT for editing, then gained 4 TBT back. It's just recalculating because you edited.
I just edited this post and did nothing.
The first 2.5 TBT is my original post. Because I edited the post, it has to recalculate how much TBT I earned from it (which is the same, since I didn't change anything). So I gained & lost another 2.5 TBT, cancelling each other out. So I didn't actually lose anything.
TL;DR: You don't lose TBT from editing posts unless you remove content.