Q: Can i block all incoming calls except for one number on an iphone 4s

Only if the carrier for your iPhone provides that service. So call them and them to block the calls.

Apps that claim do block in fact only ignore the call which all that can be done once the call is on your iPhone.

You can't actually block any calls or SMS's, but you can use Do Not Disturb:

http://support.apple.com/kb/HT5463