iphone when's it time to upgraf

[car10002]

Hi

What's the signs it's time up upgrade to iPhone 5s or 6 from the iphone 4 because the iphone 4 when it's on internet it closes out and it's getting worse and has done it 3 times tonight,

If your not in a position to upgrade what can you do to your old phone, or is there cheaper iPhones until you can upgrade like secondhand iphone 5 or something

What's average cost of secondhand iphone 5 no good spending a lot to buy time as it's cash you could put to the 6