Sounds to me like you may have damaged the proximity sensor cable when you did the repair. If your brave enough to open it again look along the cable for a tear. Specifically at the curve near where it connects to the logic board.
If you cant tell this way you can take it out to check it. Search on youtube for instructions (iphone 4s proximity sensor repair) and go for the directfix one if its still there.
Check your logic board for damage, if you cant find anything the cable is most likely torn or faulty, you will need to replace it.
Search for it on ebay, ul get 1 between 16 and 40 euro depending on where you buy it from and how long you want to wait. I order from the UK if im in a rush a part, takes about a week where i am to arrive. If not i order from Hong Kong and wait anywhere up to 4 weeks. Be sure to check if there is a customs charge wherever you buy it from.
Make sure its for the 4s and not the 4 as they are different.
Apple will know if it has been opened and your warranty will be void and also your insurance if you have any.