Well, I installed 4.0.22 over the top of the last v3, as was encouraged from the download page.
I had the same problems you had Carlos. Send button was dead. It wouldn't trigger the spell-check (I have it configured to check pre-send. Punch the spell check button .. it's dead. So, I began playing with HTML / HTML + Text and text only options and, as has already been stated, discovered that everything worked (spell-check and send email) using either text only editor.
Thinking this might have been caused by installing 4 over 3 .. I then completely un-installed (yes, I purged the registry also) and re-installed. It works! I'm now able to send email (spell-check also works) using HTML.
Something else. After the re-install and message-base restore, and before attempting to re-send an email, I ran maintenance (all options selected EXCEPT purge old messages) and it said there were problems with 22 folders! I immediately re-ran maintenance and it reported no problems with any folders. I don't know if this had any impact on 4.0.22 deciding to work sending mail using HTML.
I'm still testing to see if everything I'm used to using (I don't use all of Bats capabilities) is functioning.
What all was 4.0.7 supposed to fix? With HTML mail-send/spell-check working in 4.0.22 .. need I install it?