Although I do agree with you stateofidleness in that sending a ping first is worth doing a lot of the time, I dont quite agree with your specific argument - sending a ping first and getting a reply does not mean that sending the email will not fail half way through. All it means is that the initial connection is likely to succeed (although as dbasnett points out, just because an SMTP server returns a ping request it does not mean that the SMTP service is running and responding). So basically you could send a ping, get a reply, and the server could still die half way through you sending your 4 MB of data.
Also
Before I send a 4MB email with attachments, I'd like to know that the server is online to accept it. If it isn't, then it will get bounced as well, which is even more bandwidth.
The 4 MB of data only gets transferred after the server has accepted the initial SMTP connection and has accepted the sender and the recipient(s) specified in the email. So it would not get as far as sending any large amount of data if the server was offline to begin with.