Are some of my iphone 4 apps not going to work when ios8 comes out next week?

Are some of my iphone 4 apps not going to work when ios8 comes out next week?

No they will all work fine. Usually where you have a problem is if you can't update to the latest OS, then in a year everyone writes apps only for the latest OS, but thats still a long process. I actually wouldn't update 4 if everything works fine as that hardware is somewhat outdated now so stick with 7.

Most likely so, Then don't update your old phone to ios8

That would depend on if the app requires iOS 8 as minimum