- Hardware design is prone to reception issues
- MicroSIM card support only
- No Flash support in the web browser
- No true multitasking for all applications
- FaceTime video calls work only over Wi-Fi
- No file transfer over Bluetooth or USB Mass Storage mode
- No hardware shutter key for the camera
- No FM radio
- No stereo speakers
- No microSD card slot
- No smart dialing
- Too dependent on iTunes for loading multimedia content
- Poor loudspeaker performance
As you can see, most of the main disadvantages are simply passed from one generation to the next but - whatever iPhone you're coming from - the Number 4 will tick most of your boxes. Upgraders will be used to the shortcomings, and unbiased observers will have less points to complain against.
It just seems some features will be forever missing. The iPhone's memory isn't expandable and you can't use the thing as an external drive (this also means that files are only transferred via iTunes, again). Bluetooth has been upgraded to cover not only for music and calls but a compatible wireless keyboard too. File transfers however are a no-go.
The lack of Flash support in the Safari browser is no surprise given the Apple-Adobe feud. Luckily there's the good old YouTube app to partly make up for that but Flash games are still out of the question.