hello i have iphone 4 and i want ask about new software 4.2.1 is it stable or it has problem even that is no jb

4.2.1 is stable, but upgrading to it will mean two key things:

1) You will be upgrading your baseband to a radio baseband that cannot be unlocked. If you want to use the phone on a carrier other than the one who sold it, this will lock you out of doing it.
2) iOS 4.2.1 can be jailbroken at present, but it is a tethered jailbreak and must be connected to the computer every time the phone boots if you install any packages that load at boot time.

Aside from that, if you're NOT jailbreaking, iOS 4.2.1 is stable -- no problems. It's got a few new features, etc. When you jailbreak, there are some issues -- some packages haven't been updated and do not work, others will bootloop the phone, etc. It's not recommended to upgrade to 4.2.1 if you want to jailbreak unless it is your only choice.

Good luck!