The are a couple of possibilities here. 1) The touch screen is defective; 2) The connector is not fully seated; 3) The flex ribbon cable was torn or pinched during reassembly.

When replacing the front assembly on the iPhone 4, there are two pieces that need to be transferred to the new assembly: 1) front camera bezel and 2) the earpiece grill or cover. Look at your old front cover and see if both are still there. They will need to be gently removed, usually with a razor blade. When setting both of these into place, it is easier to put them on the phone, then set the front assembly down on top of them. Notice that the camera bezel ring has tabs on it and will fit one way only. Rotate it until it is sitting flush. Don't bother trying to reuse the adhesive on the speaker grill. Clean it off, place it shiny side up on the phone and carefully seat the front assembly.

The black spot on the LCD most likely was caused by something pressing on it just a little too hard. The spot may or may not go away. We have seen this quite a bit on cameras and LCD monitors. It is usually caused by finger pressure. It's just enough pressure to displace the liquid, but not enough to crack the screen. The camera should still be useable for the short term, but if the spot does not go away, or gets bigger, then a new screen it is.

The LCD has failed. There is not a problem with the main board. Replace the LCD and you will be good as new.