Jailbreak iPhone 4 on 7.1.2


I'm having trouble jailbreaking my iPhone 4. I've used the Pangu jailbreak many times, but doing it on such an old system on such an old phone isn't working. The "Jailbreak" button on the Pangu program just isn't enabled. I plug in the phone, and it tells me to click "Trust", which I do. But then I can't click on the Jailbreak button to start the process.

What am I doing wrong?

Thanks in advance.