I suggest you look through this site.
P-O has documented more tanks in the detail we like than anyone on the web. It is certainly the best source for M4A2 and M4A4 photos in as near as-built as is possible almost 70 years later. In many or most he has included photos of the serial stampings, so
an hour or two ought to give you a good bit of data.
From: G104@yahoogroups.com [mailto:G104@yahoogroups.com] On Behalf Of adrianharris21
Sent: Monday, March 05, 2012 2:30 PM
Subject: [G104] Re: M4A4 engine access panel shape
Thanks for the reply - the only one I've on this subject so obviously not something on which there is much documentation!
--- In G104@yahoogroups.com, TANKBARRELL@... wrote:
> Mine is rectangular and is serial 5271, September 1942, so fairly early. I
> have often wondered when the change occurred.