How can I delay delivery of mail via SMTP relay, perhaps sendmail

In your question, you should have been at least a little more specific. Anyway, I'll just make some suggestions.

If you just want to deliver mail every four hours, you need to run sendmail in queue mode only (QUEUE_MODE = "cron", in sendmail.conf) and set up a queue to run every four hours (QUEUE_INTERVAL = "4h";). I think this only applies to similar debian systems, but the principle is the same - you set the queue mode in cron (this is actually controlled by the arguments with which you run sendmail) and then you process it periodically.

If you just want to delay mail delivery, there are also several ways to do this, depending on why you want to do it. One of the popular solutions is greylisting, it only does the following: when a host connects to your MTA (sendmail, f.ex.), it gets a response with a prompt to try again after a while. A properly configured email program will simply do this — it will try to send the mail again, and eventually the message will be accepted and delivered (or forwarded). Most spambots, on the other hand, will not try to resend a message after receiving an error. If you need greylisting on sendmail you can read it here: http://www.greylisting.org/implementations/sendmail.shtml

Hope this helped at least a little.

EDIT:

Ok, now I see what you need to do. Here's a possible solution using sendmail (I've been doing sendmail for years, so :: P): you use two of them.

The first one receives mail and queues it up and (and this is important), it does NOT process the queue. The second instance of sendmail starts a separate queue, and for QUEUE_MODE, a daemon or cron (say every minute). Now you need to write an external script that moves mail from the first queue to the second after the message reaches its "age". Since the queue items are just files, this is a simple task done in a few lines, say perl (heck, a shell script can do that too). Moving queued items from queue to queue is as easy as moving files from directory to directory. Note that this method is widely used in mail handling solutions like, say, spamassassin, so its not some weirdness caused by my mind: P

Hopefully this gives you a hint or two.