Why isn’t stage manager available for IPad Air 4th Generation ?

The new Stage Manager feature is available for these models of iPad:

• iPad Air (5th generation)
• iPad Pro 11-inch (all generations)
• iPad Pro 12.9-inch (3rd generation and later)

More information here:

https://www.imore.com/how-use-stage-manager-ipad

Move, resize, and organize windows with Stage Manager on iPad - Apple Support

Feature availability is usually limited by system hardware - CPU and available RAM being critical to some advanced functionality. Models supporting Stage Manager have an M2, M1 or A12X CPU (upon which the M-series CPUs are reportedly based) - with at least 8GB RAM.

Your iPad Air4 is and will remain a very capable device. However, for the new Stage Manager feature, its particular combination of system hardware does not support this advanced functionality in its present form. While Stage Manager is not currently supported for your Air4, you still benefit from the majority of new features introduced in iPadOS.

A further example of hardware and feature compatibility is the new “hover” function associated with the Apple Pencil. The previously top-of-the range 2021 iPads Pro (with the M1 CPU) do not support this new “hover” feature - this only [currently] supported by the new 2022 models of iPad Pro (with M2). Whether or not this particular limitation is hardware or marketing related is unknown - as, somewhat confusingly, “hover” capabilities are supported by 2021 models for Accessibility features.

New capabilities will always be released - and not all will be supported by recent but older hardware. This is not unique to Apple, but is true of any computer and/or Operating System.