Which I pad do I buy for Stage TRAX 3 app

You can buy any iPad that can run at least iOS 13. Starting 2023 this will be the minimum requirements to run ST3.

Here is an overview which iPads run iOS 13: https://osxdaily.com/2019/06/04/ios-13-compatible-iphone-ipad/

peter peter do you know if stagetraxx works with 9th generation i pads im thinking og getting that or a ipad pro 10.5 inch, any advice as per how to connect it to mixer? and ios version lifespan etc thanks

See my comment above. iPad Pro 10.5 and 9th gen iPad are both fine.

peter thanks peter, the ipad air also looks ok, do i need any of that super processor power for stagetraxx or will the 9th gen with 64g memory work fine? it seems the headphone jack will be the deal maker for connecting to mixer

The 9th gen will be fine.

Hello Peter
I'm looking to buy a new Ipad, what are the minimum requirements for the current version and version 4 if you know it already ?
Thanks in advance

Minium requirement is a iOS13 capable device. Version 4 might push that requirement to iOS16 but it is too early to tell.