Pokemon-go – I can’t get Pokemon go

Your phone running Android 4.3 makes it incompatible with Pokémon Go. Your best bet is getting a new phone, since the S4 isn't getting newer updates, or using a custom ROM.

You are probably soft banned.

Soft banning occurs when the game detects that you're moving around too quickly. This can happen if:

• You use GPS spoofing to fake your current location.
• You travel in a vehicle at an excessive speed.
• You disable your GPS, travel a significant distance, then re-enable it.
• You lose data service, travel a significant distance, then re-connect.

The following are symptoms of a soft ban:

• When you throw a Pokeball at a Pokemon, it wiggles around a bit, then flees without fail.
• When you try and spin a Pokestop, it never turns purple, and you never get any items.
• You are unable to challenge or train at any gyms.

If you are soft banned, the ban will lift itself within a few hours.

Here's a video of what a soft ban looks like:

Currently, there is speculation that the soft ban mechanism functions similarly to the "bubble effect" in Ingress, the last ARG developed by Niantic. This effect is explained here:

No matter what speed you are traveling, you can perform an action as long as you haven't performed one in the last 15 minutes. Once you perform an action, a figurative "bubble" is created. This bubble expands at the rate of 60 km/h. If you travel out of this bubble, you will not be able to successfully perform actions. So, once you slow down, you have to wait for the bubble to catch up with you before you can perform actions again. After 15 minutes, the bubble goes away, and you can perform actions again even if the bubble hadn't caught up to you yet.

(For US readers, 60 km/h is approximately 37 mph.)