iPhone 4 no boot problem.. :(

Try this...

I'm posting here because I hope this helps you, and I hope it helps others who have the same problem.

I had the same symptoms, and I'll add that my phone would turn on if I plugged it into a wall charger or my car charger, and it usually would be OK reception-wise as well--but, again, only if it was plugged into a wall or my car charger and not if it was plugged into my computer. It also wouldn't sync, couldn't be restored, etc. etc. I've been through gargantuan amounts of headache over the last few days--not to mention missed business opportunities, embarrassments... lemme stop.

No, I'm gonna add one more thing... The thieves at the Apple store, some of whom are arrogant, condescending thieves, were NO help. OK, I understand they are good people who only believe what the higher ups tell them, but if that's the case, they're being told GARbage.

Sorry. Try taking off the back of the phone again, and make sure the plate that connects the battery to the rest of the phone is very well situated in its harness. I took a picture of it for you. Now if I can just figure out how to put that pic in here... I'm sure that didn't work, so I just added it as an attachment. (photo (37)) And, in case that doesn't work, I'm going to add another attachment of the other thing I adjusted. It's a screw that held down a metal loop near the top of the phone. (photo (38)).

In my case, the battery just wasn't powering the rest of the phone. That would explain why your battery worked in your partner's phone. I sincerely hope this works for you. If it doesn't, double check all connectors that could possibly be bringing juice from your battery to the rest of your phone. And since you're handy and have already voided your warranty, I'd suggest finding replacement parts--if you can diagnose a problem part--instead of just going to the Apple store to buy a new phone for $150.