Yes it's 28 days.
You have to leave at least 1 of the 4 maps not updated for 28 days (or 4 weeks) to have it moved to Graveyard before being able to upload the new one.
Alternatively, you can also either rank maps or purchase supporter to extend that submission cap, but also do note that you still have to follow the above if you filled up the cap again.
For the latter part of the question, ranking is a rather pretty complicated process than just uploading and call it a day. You can refer to
this wiki page to have a basic gist about its procedure.