You would receive 1/12th of 4% of the amount every month, either added to the value of the cd or deposited into your checking/savings acct; which way is your choice. If added, it compounds. Thats why you see some cd's show two rates, one slightly higher. The slightly higher one is the effective rate if you compound the interest into the cd.
If you took a cd for 10000 @ 4% for three months you'd have 10000 plus ((10000 *.04) / 12 * 3) in interest.
Do a google for "certificate of deposit calculator". You can plug in the amount, the term, the rate and so forth and it'll give you an 'amortization table' of the amount you'll get, by month and year.
Note the cancellation terms of the cd as well. Some have you forfeit a lot of interest...or all of it...if you cancel the cd before the term expires. Some only take a month off a 1 year or 6 months off a five year if you close it. That can make a big diff if you need the money for an emergency.
Except for 3+ year terms, a money market makes more sense.
You may go ahead and post any questions you want. Somebody will probably take the time to answer them more or less politely.