unable to donwload apps

Well, bearing in mind that you can just download apps which can support iOS 9.3.6 (the max iOS supported version by your iPhone model), see this support article:--> If you can’t download or update apps on your iPhone or iPad - Apple Support

Regards

Giulio

The problem is that your iPhone 4s is too old and the hardware on that phone is not capable of running a version of iOS that is required to be able to run a lot of the apps. You will not be able to download them as the hardware will limit what it is you can get. Unfortunately, you are going to have to upgrade your phone.