Getting hot when battery is connected

start by taking the phone apart again, and taking off the shields on the logic board. dip the whole thing in some 90%+ isopropanol and soak for a few hours and then clean with a toothbrush and more alcohol.

The heat is coming from a short on the logic board.

Your troubleshooting looks like this:

1.) Clean the board and dry--test. if not solved

2.) Apply flux and attempt to reflow the oxidized solder on components in the area that is getting hot---this is a risky DIY fix, and it may be worth it to send out for this service

3.) If still not solved, you need a board-level repair to find and replace the shorted component---this can be quite time consuming so it may not be worth it for an iPhone 4.

You did a lot of things right with this water damage repair---next time do an alcohol clean before starting the phone...the alcohol displaces the water and remove any trace minerals that would otherwise dry on the phone.

jessa