Yes, removing the battery while the iphone is powered on can cause this issue. It may take some playing around with to get it to detect the SIM again but the general idea is to try to get the "Activation" screen to come up. You can try a different SIM You can try to go to the reset menu and either reset ALL SETTINGS, or reset network settings. Do this with the SIM in. If that doesnt work try doing it without the SIM in. Only insert the sim after you see the "No SIM" message.

Which company is the iPhone 4 from? Also, which company is your SIM from? Is the SIM active? If the iphone is from the same company as the SIM and the SIM is active, then try this: Remove the SIM card from the phone. Reboot the phone. Make sure it says "Insert SIM" at the top left of screen Insert the SIM card Verify that is says "Searching" at the top left of screen. What happens now?

Sounds like you reset the phone from the settings menu, correct? Try this: Hold Volume UP, and the home key down at the same time. Now, while still holding those two keys, press and hold the power key. When you see the Samsung logo you can let go of the power and continue to hold the other two keys. You will see a screen with blue text that says "Android Recovery" at the top. Now use the volume down key to scroll down to "Wipe Data" and then use the power key to select it. On the next screen scroll down to Yes then again select it with the power key. This will hard reset your phone. When it boots up, allow upto 15 minutes for it to fully boot. Report back with what happens!

Depending on the location of the screw, you may be able to grind the screw head completely off. Then once the part you are removing is completely out, you should have enough of a screw protrusion to be able to grip it with small pliers and turn it until it comes out. Again, this is quite advanced so you would have to be comfortable with working with a Dremel tool. You can also get kits at home depot for removing stripped screws. There are ones that come with putty that you can apply to the screw head and then when it starts to harden you can push your screw driver into it to create an imprint. Once it dries the screw should come out reasonably easily.