twitter client for iPhone 3G and iOS 4.2.1 I am running iPhone 3G with iOS 4.2.1. The official client (version 4.3.1) crashes and tweetbot (version 2.4.2) does not work (http errors 4xx). Upgrade is not possible. Is there any way how to fix it or any other client? Note that I do not want to jail break my device.

I guess that it is caused by switching off of Twitter API 1.0 http://www.sitepoint.com/rip-twitter-api-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