4 Year Old and 11 month Old HELP

That sounds like an awesome plan to get the most use out the seats you've got to me. I'm going to have to replace both of my seats in 2015, when my older DS will be almost 5 and baby DS will be almost 2.

You are right that with those brands it does depend on the month they were manufactured. Graco is the only brand I know of that goes to Dec of the year, so a seat manufactured in Jan 2008 by Graco would be good until Dec 2014, for example.