Few reasons why i would take gs4 over 1020-
Notification center - u see all notifications/events in a single window, you can clear each or all easily. No way to do this in windows phone, toast notifications Always appear , no setting to turn off for messages, or otherwise. Privacy can not be ensured unless set in kid mode.
Android apps are far superior, always updated. You get plenty of apps, you can set them to update automatically for you. Unlike this wp steaming crap where you never get updates on time. -reason why we have wp app update thread in wp central.
1080p screen over 768p , more details less scrolling. Sg4 Also comes with Amoled display. Led notification lights available in sg4 which can be a good boost.
File manager, task manager. Can see app memory usage, remove apps from memory.
Multiple volume levels, individual for headphones, music, call volume, notification volume. In windows phone you only get Single setting for All whichs fail
Slow animations switching between windows in wp metro apps, which can A big irritant. There's option to reduce or turn off animation in Ndroid phones. I see windows phone hide lag by transitioning between windows due to slower animation.
If you just want to take photos, 1020 is for you. If you're a geek, I would go for sg4. All the best.