Happy news for everybody who wants to jailbreak iPhone is that it is safe. You should not worry about voiding the manufacturer's warranty after following the process of jail breaking.
Though the term is a bit scary but there is absolutely nothing to be afraid of.
Jailbreak iPhone 4 is a procedure that allows you to install some basic software in the mobile device that overrules the limitations put by Apple on the phone. Moreover, you can get rid of the installed software at anytime you wish. Apart from being risk-free it is legal as well.
The concerned authority passed a decision that iPhone 4 is a personal device and thus the owner can do anything with it. Thus, jailbreak iPhone 4 should not be a matter of concern for anybody at all. However, you need to consult an expert in order to jailbreak the device as it is not possible for anybody to do.