Its very hard to remove the the screen from the frame and is best to replace everything screen , digitizer LCD and frame .You can buy them through ebay or Amazon
https://www.youtube.com/watch?v=iS02J12I1MU
or
https://www.youtube.com/watch?v=o9AxbICPEMY
http://www.amazon.com/LSHtech-Display-Digitizer-Assembly-Replacement/dp/B0107L5LM8/ref=sr_1_5?ie=UTF8&qid=1441101093&sr=8-5&keywords=LG+G4+screen