I just got rid of my S4 for the Note 3, but mine used to do the same thing. It's just working the battery really hard, actually it can't even charge as fast as it is draining the battery. Playing youtube videos to my TV using the HDMI adapter was the same thing. If it's not melting the phone back plastic, I wouldn't think it would melt your seat, but then again, you know better than I as to how hot it's getting.
Answers & Comments
I just got rid of my S4 for the Note 3, but mine used to do the same thing. It's just working the battery really hard, actually it can't even charge as fast as it is draining the battery. Playing youtube videos to my TV using the HDMI adapter was the same thing. If it's not melting the phone back plastic, I wouldn't think it would melt your seat, but then again, you know better than I as to how hot it's getting.