Q: how to instal SKYPE on my iPHON4 .It's impossible because my iOS is 7 but SKYPE needs 8

Sorry. You can't. Skype will not run on iOS 7.1.2. The iPhone 4 can not run anything higher than 7.1.2. It's obsolete.

It's time to seriously consider upgrading to a newer phone.

Follow this: https://www.skype.com/en/download-skype/skype-for-iphone/

Under system requirements:

• iOS 8 or above.
• This version works on iPhone 4, iPhone 4S, iPhone 5, iPhone 5C, iPhone 5S and iPod touch 5th Generation.
• For users with iOS 5 or 6, Skype 4.17 for iPhone is available for download in the App Store.

The latest version can not run on an iPhone 4 because it requires IOS 8 and an iPhone 4 cannot run IOS 8.

I know. Did you read what I posted?

See the information in this support document about compatible versions of apps for older iOS. Install an app on an earlier version of iOS or OS X - Apple Support

You said

This version works on iPhone 4,

Did you not mean to?

Wasn't my own words. System req from skype.com. If they have a version that works on iOS 5 and 6, I find it hard to believe it can't run on iOS 7 unless of course their website is out of date.

Then they are wrong and you repeated their error. Because it is an error, nothing that needs IOS 8 will run on an iPhone 4, for obvious reasons.

Earlier versions (if available) should work.

Csound1 wrote:

 

Earlier versions (if available) should work.

That's what I said:

If they have a version that works on iOS 5 and 6, I find it hard to believe it can't run on iOS 7 unless of course their website is out of date.

Whatever, I just didn't want your incorrect advice left there as was, check your facts (its in the TOU) and goodbye.