Problem with restoring my iphone 4 :/

The error 21 is generally a security software thing, but you have opened the device and also used jailbreaking software on it. The 3194 error is for you trying to use custom firmware that does not belong on the device, like downgrading. You have forfeited any support by Apple or here on the forum.

Besides the drop in the pool, your bigger problems started when you tried the jailbreak, then opening it and replacing the battery and all of that. If you had come here when you first dropped it in the water, or when the first restore wouldn't work, then we could have told you to get an out of warranty replacement. Since you opened it, Apple won't offer that to you any longer either. You have unfortunately put yourself in a position that you will have to either purchase a new phone at full price, or see if you can get a subsidy from your carrier or take the device to a 3rd party repair. You are out of luck here. I'd watch who's suggestions you follow next time.

The iPhone does not have user servicable parts. If you purchased the device there from a supported reseller, they may have some way of repairing it, you would have to ask them. The software you used is jailbraking software and when you try and alter the iOS with it, it is the same thing. I'm sorry you had issues with your phone, but every time you tried something different, be it something you read on the Internet or heard on the street corner, you just made it worse, up to the point now that it is probably unrepairable. It is time to cut your loses and get another phone, even if it isn't an iPhone.

Yes, and don't listen to them about how to fix it! Good luck.

Error 3194:

This device isn't eligible for the requested build

It means that Apple has stopped signing the version of iOS that you have, try downloading the latest iOS version, then try Restore again.

Well Apple doesn't replace devices for water damage, and after you get your Idevice wet you should turn it off and put it in rice immediatley. By turning it on AND jailbreaking it you probably have permanetly damaged it. I would take it in to Apple or any Apple store near you. They might replace it since you've really done a number on it. Hope this helps!