You can find all the downloadable maps for CoD4 here: https://www.xtremeidiots.com/files/category/2-maps/
Given that there's more than 1000 of them in the repository, you could use the list of maps currently online, which changes every week, as a reference. It can be found on this forum : https://www.xtremeidiots.com/forums/forum/233-call-of-duty-4-map-rotations/
That being said, I would tend to believe that it is faster to just let the maps download ingame than to browse through the maps repository to find the ones you need for this week's rotation. Sometimes downloads do get very slow (for a reason unknown to me), but these are non-recurring occurrences - I mean, it does actually get better when you try to download the map several times in a row. So if you're stuck on the downloading screen ingame, you can either close the game and try again like Shiny said above, or you can go in the console (which key does that depends on your keyboard, I think it's usually the '~' key on american keyboards?) and type /reconnect . This will, well, reconnect you to the server, and start the download again. Do that a few times in a row if needed, it has always worked for me.