Iphone 4 Software

I need to download apps onto my iphone, specifically Yahoo mail as it takes so much time to go through the security for online login (it has to email or text my mother and she has to send me the log in code from England, so obviously time difference creates major problems). However, when I try to download the apps onto my iphone it says I have to have IOS 10 to be able to download the app. BUT YET, I cant update the software on my iphone, so it's a continuous circle of not being able to do ANYTHING.