Apple pen 1 gen and Ipad pro 4th gen don’t work together

This is correct. Why did you think it did support it?

As shown in the support document you posted from, the iPad Pro 4th gen is compatible with the Second Generation Apple Pencil.

Apple Pencil (2nd generation)

You can use Apple Pencil (2nd generation) with these iPad models: 

• iPad mini (6th generation)
• iPad Air (4th generation and later)
• iPad Pro 12.9-inch (3rd generation and later)
• iPad Pro 11-inch (1st generation and later)

Here is Apple’s definitive compatibility list for Apple Pencil and iPad:

Apple Pencil compatibility - Apple Support

iPads that support Apple Pencil will support either the first or second generation Pencil - but not both. The first and second generation Pencil use an entirely different pairing and charging interface. 

The second generation Pencil can only be used with iPad models that have the magnetic/inductive dock that is exclusively designed for the Pencil. Charging is controlled via the iPad magnetic dock.

The first generation Apple Pencil uses the physical Lightning port of the Pencil and host iPad for both pairing and charging the first generation Pencil. The first generation Pencil can also be charged using an inline charging adapter and the iPad’s Power Adapter. This pencil can only be used with compatible iPad models that have a Lightning connector; with the sole exception of the new iPad10, requiring a special adapter, iPad models that feature a USB-C port lack any means by which to pair with a first generation Pencil.

In summary, the Apple Pencil models are not interchangeable - and can only be used with the iPad that has appropriate internal hardware.

True, but sometimes it just not possible to maintain backwards compatibility.

In the case of the Pencil, the way they connect was fundamentally changed, so its physically impossible to connect the first generation Pencil to the newer USB-C equipped iPads.