It depends.
If you're doing it for fun, video-blogging, you may force the iPhone 4 to do video post-processing.
If you're doing it for pro, record on the iPhone 4 then edit it on desktop (yes you can put 'semi' word before 'pro' ^_^)
2 reasons.
1. It's a phone.
Its main function is a communication device. Rendering a video can take forever on this device & some apps are not natively multiprocessing. Anticipate a phone call while rendering. A phone call means interrupted rendering process. Not to mention the battery life. Don't try to render your edited video while far from electricity outlet. "Sorry I can't get your call, my phone is still rendering & the battery is low."
Takes 30 mins to render this 45 seconds video. 30 mins without receiving any phone call :D
And since it's a phone, you have to get used to edit in a small multitouch screen with your finger. iMovie for iPhone is great, but keep in mind that from desktop to tiny multitouch screen is another learning curve: finger editing.
2. Quality matters.
Color-correction is a critical process in video post processing. Without proper compression method, the video file will be 'damaged' due to inappropriate lossy data compression algorithm.
For instance, CinemaFX for video is the coolest color-grading app for iPhone 4. Problem is, by default it compresses the video from the Camera Roll, means that you'll get a video with blocky compressed quality - not the original.
Don't expect some Sorenson, DV-Pal or any professional video compression codecs for iOS anytime soon.
Try watch the video in HD to see the comparison.
iMovie for iPhone on the other hand, is using H.264 compression, which is good enough for final video. But not good enough to do multiple post-procs.
Is it ready for video post-processing?
"Hello? I'm shooting a video right now with my iPhone. Can I call you back?"
