twitter client for iPhone 3G and iOS 4.2.1
I can't help much with which app will work the way you want but I can direct you to list large list of compatible apps you can check out.
Support site for an app to find early apps, https://www.facebook.com/VintApps313
And VintApps421 is also coming soon.
Here is a more manual but free method,
https://sites.google.com/site/appleclubfhs/support/advice-and-articles/finding-apps-for-older-devices