how to install ios 8.0 in iphone4

That is not possible.

If you need to run iOS 8 then you need to get rid of your obsolete iPhone and replace it with a new one that can run the newer code.

how i update ios 7 to ios 8

how instal ios 8 app in ios 7

How do I down load ios8.0 for free

OMG, read the thread. You cannot download iOS 8, or install it on the iPhone 4, or install it on any device, period.

I want new update my set iPhone 4

You cannot

Read my reply post above.

I want update my iphone4

An iPhone 4 can only be updated to iOS 7.1.2, it doesn't support higher versions

It doesn't matter what you what, your iPhone 4 is not capable of being upgraded.

Hjij