Is it normal 4 days after back molar extraction, bone graft, and implant to feel a small lump in my gum where my tooth was?

What you are describing is completely normal.  After an extraction and bone graft,  the worst of the healing prices is at 3-4 days.  You are in that window so hang in there and it should be fine.  

Hello... after the bone graft is normal feel the lump that probably is bone of the doctor put. Don’t be scared of that it will pass in few days. In case it hurt you, feel so much pain and pus get in contact with your dentist. 

Without photos or xrays I couldn't say what it is. If it's soft and depressable, it's probably an abcess and should be treated as soon as possible with xrays. If it's hard and solid and you can't move it, it could be bone or the screw on top of the implant. As the gums heal and the swelling goes down, you may be able to see or feel part of the implant through the gums.