Hi so I have a iPad Air fourth generation and I have an older computer I don’t know what type of model it is but it’s an older iMac and I don’t know how to restore my iPad

The latest version of Apple iTunes, which you will need, for Macs requires a Mac that can run, at least, MacOS X 10.11.6 El Capitán, or later.

Any iMac from 2007 to early 2009 will run Mac OS X 10.11.6 El Capitán.

Later iMacs from late 2009 to 2011 can run up to macOS High Sierra, 10.13.x

Later iMacs from 2012 to present can run either from macOS Catalina, Big Sur or the up and coming macOS Monterey.

macOS Catalina is the last Mac OS version to be able to use the latest version of iTunes.

Restore your iPhone, iPad, or iPod to factory settings - Apple Support

Good Luck!

These support pages outline the available methods to restore an iPad - either from a backup, or to Factory Settings:

Restore your iPhone, iPad or iPod touch from a backup – Apple Support

Restore your iPhone, iPad or iPod to factory settings – Apple Support

Restore all content to iPad from a backup - Apple Support

You’ll need access to either a PC (with iTunes installed) or a Mac (with iTunes or Finder - as appropriate for the installed version of MacOS).

Download and use iTunes for Windows – Apple Support

Update to the latest version of iTunes – Apple Support

Use of a PC or Mac computer is unavoidable. If you don’t have access to a suitable computer of your own, that of a helpful friend or family member will suffice. Alternatively, the technicians at the Genius Bar of your local Apple Store or Apple Authorised Service Provider (AASP) will be delighted to assist.