Modify templates/views/checkout/cart_content.tpl The section at the end starting {if $checkout_add_buttons} through the end of the file displays the paypal express button on the cart page. Duplicate that section wherever you want it.
On the checkout page you *might* be able to duplicate that section into templates/views/checkout/components/steps/step_one.tpl to display it along with the login/guest/register.
Both may require some tweaking of css to display properly.
Modify templates/views/checkout/cart_content.tpl The section at the end starting {if $checkout_add_buttons} through the end of the file displays the paypal express button on the cart page. Duplicate that section wherever you want it.
Place this code in HTML editor of Payment instructions for Editing payment: PayPal Express. You will have PayPal button on step 4.