If a single message caused the problem and the email size was 4738kb (bear in mind that email size will be bigger than the attachment size - an attachment of something like 6.75MB will fill a 10MB mailbox) then there must have been other mail in the account at the time - it may be worth checking your trash folder as this counts towards your total.
If you want to keep an eye on exactly what you have in your webmail folders then there's an excellent freeware utility available called
IMAPsize (just point it towards imap.ntlworld.com)