Device won't power on. loops at apple logo.

1. Power button is stuck. *not very suprising as this was a common problem*

The small black circle on the power button has gone loose causing it to not click, You will either need to replace the power button and sensor cable as shown here:

iPhone 4S Power and Sensor Cable Replacement

Or, replace just the yellow sticker part with the circle that is on top of it with a part like this:

https://www.ebay.com.au/itm/Apple-iPhone...

I always replace the power buttons on the 4 and 4s like this, I remove the old yellow plastic and put the new yellow plastic on, if you do end up doing it like this make sure you don't lose the silver metal piece that goes under the yellow piece, this way is a lot quicker to do and less hassle.

2. device won't power on. *Loops at apple logo. never shows battery flat symbol*

While the phone cycles the apple logo, the phone will probably have the apple logo go off immediately when you unplug the charging cable. If this is the case the battery is either not connected properly or faulty.

You can get a replacement battery from here:

iPhone 4S Battery

Guide to replace the battery:

iPhone4s 电池更换