Here's "How to Root ASUS Zenfone 4"
(I don't know in case device can be unrooted after rooted from method from the link).
1. is it easy to root z4?
-- Yes, In case you read(know what you are doing) and follow carefully. In short, not impossible.
2. like no problem in rooting my phone?
-- I don't think any dev take responsibility for any type of device damage after following his/her instructions. But mostly damages not happen. so always make sure by reading comments, possible that someone have replied success message.
3. once i root my z4, is there a recommended bloatware list i can freeze?
-- "Recommended", yep maybe in case someone have made list. but in simple, things you don't use and are not part of official firmware and by removing them it does not effect device OS. then it's fine to stop them or lock them from being loading in startup/boot or completely uninstalling them. there are apps to make this work easy. (i'm not recommending anything, just sharing for idea.)
4. is there other way to make the battery last longer?
-- Yes. there are threads on this platform.
5. and when will kitkat upgrade available to zenfone 4?
-- I don't know in case there will be Official upgrades or not. But maybe there are Custom ROMs on internet.
6. any unrooting guide here?
-- already answered. Other than last answer regarding this question, there are ways to unroot device with different tools type. but not sure about method of which i posted link above.
i just want to upgrade first my z4 before i root it.
-- I think there is option for upgrade/update in Phone setting "About phone" and in case upgrade/update is available then it will be Official.
Do not try anything on your device Until you know what you are doing. always make full Backup of your device before doing stuff like "Root/CWM(Custom Recovery/Custom ROM). don't take my answers as perfect, make more search, wait for more replies.... until you get enough information and are sure that you understood well, what you learned etc....
hope, answers helped you a little.