Cordova app element movement on iphone 4 only

I am very new to cordova apps and developing for phones so apologies if this is a simple fix. I have a sign in page which consists of a box for email and a box for password and then the sign in button. Below all of this right at the very bottom of the screen is a further button which links to a buy something page. This all works fine except when i view the app on an iphone 4s. In this case the link to the buy something page floats up and places itself over the password field. I have tried changing the css positioning to relative but this then messes the app up in iphone 5 and 6. I then tried to implement a z count which did sort of work but also affected other functionality. The relative bit of the css is below.

&.start-button {
    bottom: 0px;
    position: absolute;
    width: 100%;
    background-color: transparent;
    left: 0px;
    border-top: 1px solid #ddd;