While I've never worked on an iPhone 4, I've seen problems like this before with other devices. It sounds to me like an electrical short.
After reviewing the guides, it seems likely that one or more of the cables near the top of the phone (camera, power, etc) are not seated properly. It is also mentioned repeatedly that oils from one's fingers can cause wireless connectivity issues, so be sure to use a de-greaser, as recommended.
I've dealt with connectors like those on laptops and they can be temperamental if you're not careful. Always make double- and triple-sure that they are seated properly and that no foreign objects (crumbs, dust, metal filings) might have made their way into the sockets. Use a little compressed air if you have to (just keep any small parts secured when you do!)
(Edited after I actually researched. Sorry about that!)
I hope this helps!