iphone 4 burning HOT after battery swap and now draining fast

Hello,

I'm a DIY'er and very cheap. This is my first blog post ever. Excuse my unawareness. iphone out of warranty.

Problem: My iphone burns hot when charging or when the screen is on.

Cause: I was replacing a screen on my SISTERS iphone, used my own personal battery in my iphone to boot her phone to make sure i didnt buy a dud.

But keep in mind both were ATT iphone 4 models. Both with the same IOS.

I've replaced a screen before no problem. BUT this time the phone wouldnt fire up. So to make sure I didnt buy a dud, i removed the battery from my personal phone, placed it in my SISTERS iphone and bam it worked. Yay! Her iphone is working. So i removed my battery and reassembled her phone with her battery. Still working just fine.

Now my phone is in need of repair. This is what i get for trying to help. Again, my iphone burns hot only when charging or when the screen is on. Battery is depleted in 1.5hrs.

FAILED Attempts to fix by:

Re-seatting the battery 10x plus.

Swapped batterries several times again.

Tried reconditioning.

Blew air on the charging pins.

Tried all the disable/delete mail accounts, icloud, restrictions, even airplane mode.

Reset Network settings, then tried All settings, then tried erase all and restored with a NEW setup, no backups.

Upgraded to IOS 6.0.1.

Now im just going to completely disassemble/reassemble my phone out of desperation.

I think all that may be left is possibly i caused a short. My iphone screen has two hairline cracks, but un-noticable by touch and also touch is working 100%. I think maybe somehow the screen is now causing the iphone to overwork itself. IDK how bcuz i lived with these 2 cracks for abt a year now till the recent battery swap.

Im undecided if i should try my sisters screen on my phone, bcuz i dont want anymore problems.

I dont want to buy a new phone. Any help would be much appreciated.