To block numbers on your iPhone 4s, go the phone settings and select the block number option. You can then add the number you want to block from calling or texting you. After adding the number, select save and the number will be blocked.
There is a pretty good workaround using the Do Not Disturb setting.. Put all your most active telephone numbers into your phone's Favorites. Then, under Notifications, turn on the Do Not Disturb setting and activate Allow Calls From ....Favorites. Then only the phone numbers from Favorites will ring -- and the Unknown and Private Callers will not. You will still see all the other calls come through if your Phone settings are also turned on in Notifications, along with Messages, etc. Also allow them to be seen on a Locked Screen. Not perfect, but keeps the Unknowns from ringing and bothering you, while still letting the most important calls audibly through!!!!
I have an I-phone 4s, I took the phone #s (that happened to be high spam alerts) typed them in as if they were a contact. I listed them both under one contact. Go to Settings, Phone, Blocked, Click on Add New and click on the contact that you have already added with your numbers. I haven't received calls from them since. It's too bad you have to setup a Contact, and can't just type in the numbers.
If your iPhone 4s carrier is Verizon, you can block up to 5 phone calls at no charge for 90 days. Simply go to www.verizonwireless.com Log into your account with your cell phone number and password. Click on the “View Usage” tab under “Manage” on the left of your screen. Then, click on "Managing Family Safeguards & Controls" tab on the left of your screen. Enter the phone number you want to be blocked, click “Submit” and it will take effect fairly promptly. You may want to resubmit the request again before the block expires before 90 days. I found this to be the simplest and most effective solution to blocking unwanted calls.