no its not safe. It's dangerous, if the iphone 4 naked comes into contact with air, it could explode.
It's a known issue and there's been an internal memo sent throughout apple for the Genius people to not accept broken iphones that are naked.
I'm joking. If you're a clutz and drop **** all the time, then no it's not a good idea. It's still safe though..just not so much for your iphone.
If you're super OCD about your electronics, it's still not a good idea, because it'll get scratched, and you'll feel bad, and it'll nag at you all day errday. it's still safe though...just not so much for your mental health.
If you don't give a rats ass about how your phone looks like because really..it's just a freaking phone. then it's a good idea to not spend excessive money to protect a product that's devaluing on a day to day basis. and it's safe 😉.