Despite the battery replacement it’s really simple, in my iPhone 4 there was a totally different connector for the battery (I’ll post a few photos). Wider, only one screw but at the top, with a small contact clip screwed together with the battery connector, and which flies off once you remove the battery connector. By the way the battery provided from ifixit did not fit into neither any of the screw holes nor in the socket. The socket is in a different position compared to that in the pictures here and the battery connector can’t reach at the same time the socket and it’s seat. Sadly I have to give up… 🥲