I have a iPad four and I'm trying to get the latest software update which is 13.1 and I can't get it on my ipad

The only way that you’ll be able to enjoy the latest version of iPadOS is to replace your iPad with a newer model. iPad4 cannot be updated beyond iOS 10.3.3/4.

Update support for iPad4 ended September 2017. The final iOS releases for this model were iOS 10.3.3 (WiFi Only models) and iOS 10.3.4 (WiFi & Cellular). iPad4 cannot be updated to any later major version of iOS as it lacks the minimum hardware requirements needed to run more recent versions.

Many App Developers have also dropped support for older devices and iOS versions preceding iOS11/12/13 - and have withdrawn older versions of their Apps from the Apple App Store. Withdrawal of Apps supporting older models of iPad is making their utility and usefulness significantly more limited. Once compatible versions Apps are withdrawn, you’ll not be able (re)download or (re)install them.

If Apps that you need to use now have higher minimum iOS-version requirements, your only option is to replace your iPad with newer model. Even the basic current iPad7 supports current iPadOS 13.x and soon to be released iPadOS 14 - and has hardware specifications and capabilities that will vastly outperform your old iPad4.

I hope this information is helpful - if only in clearly stating stating the limitations of your iPad and potential options.

The last update for the iPad 4th generation was iOS 10.3.3 (or 10.3.4 if it has cellular).