There is no minimum on cruise length as far as getting credits. If you have four cruises - even if they were all 3-day cruises - you still get a credit for each. But they do take their time in putting the credits in your account. It took RCI over 4 months to give me the credits for our cruise on the Liberty Of The Seas last summer. We earned two credits since we had a suite on that cruise. But eventually my account was updated. Your fourth credit should be showing up eventually.
Making platinum level is nice since you get priority embarkation and a separate debarkation lounge, as well as nicer coupons in your Crown & Anchor coupon booklets.